Diffractive devices based on cholesteric liquid crystal

ABSTRACT

Examples of diffractive devices comprise a cholesteric liquid crystal (CLC) layer comprising a plurality of chiral structures, wherein each chiral structure comprises a plurality of liquid crystal molecules that extend in a layer depth direction by at least a helical pitch and are successively rotated in a first rotation direction. Arrangements of the liquid crystal molecules of the chiral structures vary periodically in a lateral direction perpendicular to the layer depth direction to provide a diffraction grating. The diffractive devices can be configured to reflect light having a particular wavelength range and sense of circular polarization. The diffractive devices can be used in waveguides and imaging systems in augmented or virtual reality systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/835,108, filed Dec. 7, 2017, entitled DIFFRACTIVE DEVICES BASED ONCHOLESTERIC LIQUID CRYSTAL, which claims the benefit of priority to U.S.Provisional Patent Application No. 62/431,752, filed Dec. 8, 2016,entitled “DIFFRACTIVE DEVICES BASED ON CHOLESTERIC LIQUID CRYSTAL,” andto U.S. Provisional Patent Application No. 62/431,745, filed Dec. 8,2016, entitled “DIFFRACTIVE DEVICES BASED ON CHOLESTERIC LIQUIDCRYSTAL;” the contents of all of which are hereby incorporated byreference herein in their entireties.

BACKGROUND Field

The present disclosure relates to display systems and, moreparticularly, to augmented reality display systems comprisingdiffractive devices based on cholesteric liquid crystal.

Description of the Related Art

Modern computing and display technologies have facilitated thedevelopment of systems for so called “virtual reality” or “augmentedreality” experiences, wherein digitally reproduced images or portionsthereof are presented to a user in a manner wherein they seem to be, ormay be perceived as, real. A virtual reality, or “VR”, scenariotypically involves presentation of digital or virtual image informationwithout transparency to other actual real-world visual input; anaugmented reality, or “AR”, scenario typically involves presentation ofdigital or virtual image information as an augmentation to visualizationof the actual world around the user. A mixed reality, or “MR”, scenariois a type of AR scenario and typically involves virtual objects that areintegrated into, and responsive to, the natural world. For example, inan MR scenario, AR image content may be blocked by or otherwise beperceived as interacting with objects in the real world.

Referring to FIG. 1, an augmented reality scene 1 is depicted wherein auser of an AR technology sees a real-world park-like setting 1100featuring people, trees, buildings in the background, and a concreteplatform 1120. In addition to these items, the user of the AR technologyalso perceives that he “sees” “virtual content” such as a robot statue1110 standing upon the real-world platform 1120, and a cartoon-likeavatar character 1130 flying by which seems to be a personification of abumble bee, even though these elements 1130, 1110 do not exist in thereal world. Because the human visual perception system is complex, it ischallenging to produce an AR technology that facilitates a comfortable,natural-feeling, rich presentation of virtual image elements amongstother virtual or real-world imagery elements.

Systems and methods disclosed herein address various challenges relatedto AR and VR technology.

SUMMARY

In an aspect, a diffraction grating comprises a cholesteric liquidcrystal (CLC) layer comprising a plurality of chiral structures, whereineach chiral structure comprises a plurality of liquid crystal moleculesthat extend in a layer depth direction by at least a helical pitch andare successively rotated in a first rotation direction. The helicalpitch is a length in the layer depth direction corresponding to a netrotation angle of the liquid crystal molecules of the chiral structuresby one full rotation in the first rotation direction. The arrangementsof the liquid crystal molecules of the chiral structures varyperiodically in a lateral direction perpendicular to the layer depthdirection.

In another aspect, a head-mounted display device (HMD) is configured toproject light to an eye of a user to display augmented reality imagecontent. The HMD comprises a head-mounted display device comprising aframe configured to be supported on a head of the user. The HMDcomprises a display disposed on the frame, where at least a portion ofthe display comprises one or more waveguides. The one or more waveguidesare transparent and are disposed at a location in front of the user'seye when the user wears the head-mounted display device such that thetransparent portion transmits light from a portion of an environment infront of the user to the user's eye to provide a view of the portion ofthe environment in front of the user. The display further comprising oneor more light sources and at least one diffraction grating configured tocouple light from the light sources into the one or more waveguides orto couple light out of the one or more waveguides, wherein the at leastone diffraction grating comprises a diffraction grating according toaspects described elsewhere in the specification.

In another aspect, a wave-guiding device comprises one or morecholesteric liquid crystal (CLC) layers each comprising a plurality ofchiral structures, wherein each chiral structure comprises a pluralityof liquid crystal molecules that extend in a layer depth direction andare successively rotated in a first rotation direction, and whereinarrangements of the liquid crystal molecules of the chiral structuresvary periodically in a lateral direction perpendicular to the layerdepth direction such that the one or more CLC layers are configured toBragg-reflect incident light. One or more waveguides are formed over theone or more CLC layers and are configured to optically coupleBragg-reflected light such that the Bragg-reflected light travels in alateral direction perpendicular to the layer depth direction by totalinternal reflection (TIR). The one or more CLC layers and the one ormore waveguides are configured to be in the same optical path.

In another aspect, a wavelength-selective cholesteric liquid crystalreflector (CLCR) comprises one or more cholesteric liquid crystal (CLC)layers each comprising a plurality of chiral structures, wherein eachchiral structure comprises a plurality of liquid crystal molecules thatextend in a layer depth direction and are successively rotated in afirst rotation direction. Arrangements of the liquid crystal moleculesof the chiral structures vary periodically in a lateral directionperpendicular to the layer depth direction such that the one or more CLClayers are configured to substantially Bragg-reflect a first incidentlight having a first wavelength while substantially transmitting asecond incident light having a second wavelength.

In another aspect, a head mounted display (HMD) configured to be worn ona head of a user comprises a frame comprising a pair of ear stems; apair of optical elements supported by the frame such that each of thepair of optical elements is capable of being disposed forward of an eyeof the user; a forward-facing imager mounted to one of the pair of earstems; and a cholesteric liquid crystal (CLC) off-axis mirror comprisingone or more cholesteric liquid crystal (CLC) layers each comprising aplurality of chiral structures. Each chiral structure comprises aplurality of liquid crystal molecules that extend in a layer depthdirection and are successively rotated in a first rotation direction,wherein arrangements of the liquid crystal molecules of the chiralstructures vary periodically in a lateral direction perpendicular to thelayer depth direction such that the one or more CLC layers areconfigured to Bragg-reflect incident light. The cholesteric liquidcrystal (CLC) off-axis mirror is disposed in or on one of the pair ofoptical elements and configured to reflect infrared light toward theforward-facing imager that is configured to receive the infrared lightreflected by the reflective element.

In another aspect, a wave-guiding device comprises one or morecholesteric liquid crystal (CLC) layers each comprising a plurality ofchiral structures, wherein each chiral structure comprises a pluralityof liquid crystal molecules that extend in a layer depth direction andare successively rotated in a first rotation direction, whereinarrangements of the liquid crystal molecules of the chiral structuresvary periodically in a lateral direction perpendicular to the layerdepth direction such that the one or more CLC layers are configured toBragg-reflect incident light. The wave-guiding device additionallyincludes one or more waveguides formed over the one or more CLC layersand configured to optically couple Bragg-reflected light from the one ormore CLC layers such that the Bragg-reflected light travels in a lateraldirection perpendicular to the layer depth direction by total internalreflection (TIR). The wave-guiding device is configured to have a fieldof view (FOV), within which a diffraction efficiency is greater than25%, which exceeds 20°.

In yet another aspect, a display device comprises a waveguide and anincoupling optical element formed on the waveguide. The incouplingoptical element is configured to incouple light incident thereon into afirst side of the waveguide, wherein the incoupling optical element andthe waveguide are configured such that light in-coupled into thewaveguide propagates in the wave guide in an in-plane direction of thewaveguide by total internal reflection (TIR). The display deviceadditionally comprises an outcoupling optical element formed on thewaveguide and configured to outcouple light incident thereon from thewaveguide. The light out-coupling element comprises a cholesteric liquidcrystal (CLC) layer comprising a plurality of chiral structures, whereineach of the chiral structures comprises a plurality of liquid crystalmolecules that extend in a layer depth direction of the CLC layer andare successively rotated in a first rotation direction, whereinarrangements of the liquid crystal molecules of the chiral structuresvary periodically in a lateral direction perpendicular to the layerdepth direction such that the one or more CLC layers are configured toBragg-reflect light incident thereon from the waveguide towards thefirst side.

Details of one or more implementations of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings, and the claims. Neitherthis summary nor the following detailed description purports to defineor limit the scope of the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a user's view of augmented reality (AR) through an ARdevice.

FIG. 2 illustrates an example of wearable display system.

FIG. 3 illustrates a conventional display system for simulatingthree-dimensional imagery for a user.

FIG. 4 illustrates aspects of an approach for simulatingthree-dimensional imagery using multiple depth planes.

FIGS. 5A-5C illustrate relationships between radius of curvature andfocal radius.

FIG. 6 illustrates an example of a waveguide stack for outputting imageinformation to a user.

FIG. 7 illustrates an example of exit beams outputted by a waveguide.

FIG. 8 illustrates an example of a stacked waveguide assembly in whicheach depth plane includes images formed using multiple differentcomponent colors.

FIG. 9A illustrates a cross-sectional side view of an example of a setof stacked waveguides that each includes an incoupling optical element.

FIG. 9B illustrates a perspective view of an example of the plurality ofstacked waveguides of FIG. 9A.

FIG. 9C illustrates a top-down plan view of an example of the pluralityof stacked waveguides of FIGS. 9A and 9B.

FIG. 10 illustrates a cross-sectional side view of an example of acholesteric liquid crystal diffraction grating (CLCG) having a pluralityof uniform chiral structures.

FIG. 11 illustrates a cross-sectional side view of an example of a CLCGhaving differently arranged chiral structures in a lateral direction.

FIG. 12 illustrates a cross-sectional side view of an example of a CLClayer configured for Bragg reflection at an off-axis incident angle.

FIG. 13A illustrates a cross-sectional side view of an example of a CLClayer having a first helical pitch and configured for Bragg-reflectionat a first off-axis incident angle.

FIG. 13B illustrates a cross-sectional side view of an example of a CLClayer having a second helical pitch and configured for Bragg-reflectionat a second off-axis incident angle.

FIG. 13C illustrates a cross-sectional side view of an example of a CLCGincluding CLC layers of FIGS. 13A and 13B having different helicalpitches in a stacked configuration for Bragg-reflection at a pluralityof off-axis incident angles and high diffraction bandwidth.

FIG. 14 illustrates a cross-sectional side view of an example of a CLCGincluding a CLC layer having vertical regions with different helicalpitches along a depth direction for Bragg-reflection at a plurality ofoff-axis incident angles and high diffraction bandwidth.

FIG. 15 illustrates a cross-sectional side view of an example of a CLCGincluding a CLC layer having lateral regions with different helicalpitches along a lateral direction for spatially varyingBragg-reflection.

FIG. 16 illustrates an example of an optical wave-guiding devicecomprising a waveguide coupled to a CLCG and configured to propagatelight by total internal reflection (TIR).

FIG. 17A illustrates an example of an optical wave-guiding devicecomprising a waveguide coupled to a CLCG and configured to selectivelypropagate light having a wavelength by total internal reflection (TIR).

FIG. 17B illustrates an example of a plurality of optical wave-guidingdevices in the same optical path, each comprising a waveguide coupled toa CLCG and configured to selectively propagate light having a wavelengthby total internal reflection (TIR).

FIG. 17C illustrates an example of a plurality of optical wave-guidingdevices in the same optical path, each comprising a waveguide coupled toa CLCG and configured to selectively propagate light having a wavelengthby total internal reflection (TIR).

FIG. 18 illustrates an example of an optical wave-guiding devicecomprising a common waveguide coupled to a plurality of CLCGs andconfigured to selectively propagate light having a plurality ofwavelengths by total internal reflection (TIR).

FIG. 19 illustrates an example of an optical wave-guiding devicecomprising a waveguide coupled to a CLCG and configured to propagatelight by total internal reflection (TIR).

FIG. 20 illustrates an example of an optical wave-guiding devicecomprising a waveguide coupled to a CLCG and a polarization convertingreflector, where the CLCG is configured to receive incident light andthe waveguide is configured to propagate light Bragg-reflected from theCLCG by total internal reflection (TIR).

FIG. 21A illustrates the optical wave-guiding device of FIG. 20, wherethe CLCG is configured to receive incident light that is linearlypolarized or unpolarized, and where the waveguide is configured topropagate light Bragg-reflected from the CLCG and light reflected by thereflector by total internal reflection (TIR).

FIG. 21B illustrates the optical wave-guiding device of FIG. 20, wherethe CLCG configured to receive incident light that is polarized intoorthogonal elliptical or circular polarized light beams, and where thewaveguide is configured to propagate light Bragg-reflected from the CLCGand light reflected by the reflector by total internal reflection (TIR).

FIG. 22A illustrates an example of an optical wave-guiding devicecomprising a plurality of CLC layers coupled to a common waveguide,including a first CLC layer having chiral structures having a firstrotation direction and a second CLC layer having chiral structureshaving a second rotation direction opposite to the first rotationdirection, under a condition in which the incident light beam islinearly polarized or unpolarized.

FIG. 22B illustrates the optical wave-guiding device of FIG. 22A, undera condition in which the incident light is polarized into orthogonalelliptical or circular polarized light beams.

FIG. 22C illustrates an example of an optical wave-guiding devicecomprising a plurality of CLC layers coupled to a common waveguideinterposed between two CLC layers, including a first CLC layer havingchiral structures having a first rotation direction and a second CLClayer having chiral structures having a second rotation directionopposite to the first rotation direction, under a condition in which theincident light beam is linearly polarized or unpolarized.

FIG. 23 illustrates an example of an imaging system comprising aforward-facing camera configured to images a wearer's eye using acholesteric liquid crystal (CLC) off-axis mirror.

FIGS. 24A-24F illustrate examples of imaging systems comprising aforward-facing camera configured to images a wearer's eye using a CLCoff-axis mirror.

FIGS. 24G and 24H illustrate examples of imaging systems comprising aforward-facing camera configured to images a wearer's eye using adiffractive optical element comprising a plurality of segments includingone more CLC off-axis mirrors, where each of the segments can havedifferent optical properties.

FIG. 25 illustrates an example optical wave-guiding device optimized fora wide range of field of view, comprising a waveguide coupled to a CLCGand configured to propagate light by total internal reflection (TIR).

FIG. 26 illustrates an example optical wave-guiding device configured asan outcoupling optical element, comprising a waveguide coupled to a CLCGand configured to propagate light by total internal reflection (TIR).

Throughout the drawings, reference numbers may be re-used to indicatecorrespondence between referenced elements. The drawings are provided toillustrate example embodiments described herein and are not intended tolimit the scope of the disclosure.

DETAILED DESCRIPTION

AR systems may display virtual content to a user, or viewer, while stillallowing the user to see the world around them. Preferably, this contentis displayed on a head-mounted display, e.g., as part of eyewear, thatprojects image information to the user's eyes. In addition, the displaymay also transmit light from the surrounding environment to the user'seyes, to allow a view of that surrounding environment. As used herein,it will be appreciated that a “head-mounted” display is a display thatmay be mounted on the head of a viewer.

FIG. 2 illustrates an example of wearable display system 80. The displaysystem 80 includes a display 62, and various mechanical and electronicmodules and systems to support the functioning of that display 62. Thedisplay 62 may be coupled to a frame 64, which is wearable by a displaysystem user or viewer 60 and which is configured to position the display62 in front of the eyes of the user 60. The display 62 may be consideredeyewear in some embodiments. In some embodiments, a speaker 66 iscoupled to the frame 64 and positioned adjacent the ear canal of theuser 60 (in some embodiments, another speaker, not shown, is positionedadjacent the other ear canal of the user to provide for stereo/shapeablesound control). In some embodiments, the display system may also includeone or more microphones 67 or other devices to detect sound. In someembodiments, the microphone is configured to allow the user to provideinputs or commands to the system 80 (e.g., the selection of voice menucommands, natural language questions, etc.), and/or may allow audiocommunication with other persons (e.g., with other users of similardisplay systems The microphone may further be configured as a peripheralsensor to continuously collect audio data (e.g., to passively collectfrom the user and/or environment). Such audio data may include usersounds such as heavy breathing, or environmental sounds, such as a loudbang indicative of a nearby event. The display system may also include aperipheral sensor 30 a, which may be separate from the frame 64 andattached to the body of the user 60 (e.g., on the head, torso, anextremity, etc. of the user 60). The peripheral sensor 30 a may beconfigured to acquire data characterizing the physiological state of theuser 60 in some embodiments, as described further herein. For example,the sensor 30 a may be an electrode.

With continued reference to FIG. 2, the display 62 is operativelycoupled by communications link 68, such as by a wired lead or wirelessconnectivity, to a local data processing module 70 which may be mountedin a variety of configurations, such as fixedly attached to the frame64, fixedly attached to a helmet or hat worn by the user, embedded inheadphones, or otherwise removably attached to the user 60 (e.g., in abackpack-style configuration, in a belt-coupling style configuration).Similarly, the sensor 30 a may be operatively coupled by communicationslink 30 b, e.g., a wired lead or wireless connectivity, to the localprocessor and data module 70. The local processing and data module 70may comprise a hardware processor, as well as digital memory, such asnon-volatile memory (e.g., flash memory or hard disk drives), both ofwhich may be utilized to assist in the processing, caching, and storageof data. The data include data a) captured from sensors (which may be,e.g., operatively coupled to the frame 64 or otherwise attached to theuser 60), such as image capture devices (such as cameras), microphones,inertial measurement units, accelerometers, compasses, GPS units, radiodevices, gyros, and/or other sensors disclosed herein; and/or b)acquired and/or processed using remote processing module 72 and/orremote data repository 74 (including data relating to virtual content),possibly for passage to the display 62 after such processing orretrieval. The local processing and data module 70 may be operativelycoupled by communication links 76, 78, such as via a wired or wirelesscommunication links, to the remote processing module 72 and remote datarepository 74 such that these remote modules 72, 74 are operativelycoupled to each other and available as resources to the local processingand data module 70. In some embodiments, the local processing and datamodule 70 may include one or more of the image capture devices,microphones, inertial measurement units, accelerometers, compasses, GPSunits, radio devices, and/or gyros. In some other embodiments, one ormore of these sensors may be attached to the frame 64, or may bestandalone structures that communicate with the local processing anddata module 70 by wired or wireless communication pathways.

With continued reference to FIG. 2, in some embodiments, the remoteprocessing module 72 may comprise one or more processors configured toanalyze and process data and/or image information. In some embodiments,the remote data repository 74 may comprise a digital data storagefacility, which may be available through the internet or othernetworking configuration in a “cloud” resource configuration. In someembodiments, the remote data repository 74 may include one or moreremote servers, which provide information, e.g., information forgenerating augmented reality content, to the local processing and datamodule 70 and/or the remote processing module 72. In some embodiments,all data is stored and all computations are performed in the localprocessing and data module, allowing fully autonomous use from a remotemodule.

The perception of an image as being “three-dimensional” or “3-D” may beachieved by providing slightly different presentations of the image toeach eye of the viewer. FIG. 3 illustrates a conventional display systemfor simulating three-dimensional imagery for a user. Two distinct images5, 7—one for each eye 4, 6—are outputted to the user. The images 5, 7are spaced from the eyes 4, 6 by a distance 10 along an optical orz-axis parallel to the line of sight of the viewer. The images 5, 7 areflat and the eyes 4, 6 may focus on the images by assuming a singleaccommodated state. Such systems rely on the human visual system tocombine the images 5, 7 to provide a perception of depth and/or scalefor the combined image.

It will be appreciated, however, that the human visual system is morecomplicated and providing a realistic perception of depth is morechallenging. For example, many viewers of conventional “3-D” displaysystems find such systems to be uncomfortable or may not perceive asense of depth at all. Without being limited by theory, it is believedthat viewers of an object may perceive the object as being“three-dimensional” due to a combination of vergence and accommodation.Vergence movements (i.e., rotation of the eyes so that the pupils movetoward or away from each other to converge the lines of sight of theeyes to fixate upon an object) of the two eyes relative to each otherare closely associated with focusing (or “accommodation”) of the lensesand pupils of the eyes. Under normal conditions, changing the focus ofthe lenses of the eyes, or accommodating the eyes, to change focus fromone object to another object at a different distance will automaticallycause a matching change in vergence to the same distance, under arelationship known as the “accommodation-vergence reflex,” as well aspupil dilation or constriction. Likewise, a change in vergence willtrigger a matching change in accommodation of lens shape and pupil size,under normal conditions. As noted herein, many stereoscopic or “3-D”display systems display a scene using slightly different presentations(and, so, slightly different images) to each eye such that athree-dimensional perspective is perceived by the human visual system.Such systems are uncomfortable for many viewers, however, since they,among other things, simply provide a different presentation of a scene,but with the eyes viewing all the image information at a singleaccommodated state, and work against the “accommodation-vergencereflex.” Display systems that provide a better match betweenaccommodation and vergence may form more realistic and comfortablesimulations of three-dimensional imagery contributing to increasedduration of wear and in turn compliance to diagnostic and therapyprotocols.

FIG. 4 illustrates aspects of an approach for simulatingthree-dimensional imagery using multiple depth planes. With reference toFIG. 4, objects at various distances from eyes 4, 6 on the z-axis areaccommodated by the eyes 4, 6 so that those objects are in focus. Theeyes (4 and 6) assume particular accommodated states to bring into focusobjects at different distances along the z-axis. Consequently, aparticular accommodated state may be said to be associated with aparticular one of depth planes 14, with has an associated focaldistance, such that objects or parts of objects in a particular depthplane are in focus when the eye is in the accommodated state for thatdepth plane. In some embodiments, three-dimensional imagery may besimulated by providing different presentations of an image for each ofthe eyes 4, 6, and also by providing different presentations of theimage corresponding to each of the depth planes. While shown as beingseparate for clarity of illustration, it will be appreciated that thefields of view of the eyes 4, 6 may overlap, for example, as distancealong the z-axis increases. In addition, while shown as flat for ease ofillustration, it will be appreciated that the contours of a depth planemay be curved in physical space, such that all features in a depth planeare in focus with the eye in a particular accommodated state.

The distance between an object and the eye 4 or 6 may also change theamount of divergence of light from that object, as viewed by that eye.FIGS. 5A-5C illustrates relationships between distance and thedivergence of light rays. The distance between the object and the eye 4is represented by, in order of decreasing distance, R1, R2, and R3. Asshown in FIGS. 5A-5C, the light rays become more divergent as distanceto the object decreases. As distance increases, the light rays becomemore collimated. Stated another way, it may be said that the light fieldproduced by a point (the object or a part of the object) has a sphericalwavefront curvature, which is a function of how far away the point isfrom the eye of the user. The curvature increases with decreasingdistance between the object and the eye 4. Consequently, at differentdepth planes, the degree of divergence of light rays is also different,with the degree of divergence increasing with decreasing distancebetween depth planes and the viewer's eye 4. While only a single eye 4is illustrated for clarity of illustration in FIGS. 5A-5C and otherfigures herein, it will be appreciated that the discussions regardingeye 4 may be applied to both eyes 4 and 6 of a viewer.

Without being limited by theory, it is believed that the human eyetypically can interpret a finite number of depth planes to provide depthperception. Consequently, a highly believable simulation of perceiveddepth may be achieved by providing, to the eye, different presentationsof an image corresponding to each of these limited number of depthplanes. The different presentations may be separately focused by theviewer's eyes, thereby helping to provide the user with depth cues basedon the accommodation of the eye required to bring into focus differentimage features for the scene located on different depth plane and/orbased on observing different image features on different depth planesbeing out of focus.

FIG. 6 illustrates an example of a waveguide stack for outputting imageinformation to a user. A display system 1000 includes a stack ofwaveguides, or stacked waveguide assembly, 1178 that may be utilized toprovide three-dimensional perception to the eye/brain using a pluralityof waveguides 1182, 1184, 1186, 1188, 1190. In some embodiments, thedisplay system 1000 is the system 80 of FIG. 2, with FIG. 6schematically showing some parts of that system 80 in greater detail.For example, the waveguide assembly 1178 may be part of the display 62of FIG. 2. It will be appreciated that the display system 1000 may beconsidered a light field display in some embodiments.

With continued reference to FIG. 6, the waveguide assembly 1178 may alsoinclude a plurality of features 1198, 1196, 1194, 1192 between thewaveguides. In some embodiments, the features 1198, 1196, 1194, 1192 maybe one or more lenses. The waveguides 1182, 1184, 1186, 1188, 1190and/or the plurality of lenses 1198, 1196, 1194, 1192 may be configuredto send image information to the eye with various levels of wavefrontcurvature or light ray divergence. Each waveguide level may beassociated with a particular depth plane and may be configured to outputimage information corresponding to that depth plane. Image injectiondevices 1200, 1202, 1204, 1206, 1208 may function as a source of lightfor the waveguides and may be utilized to inject image information intothe waveguides 1182, 1184, 1186, 1188, 1190, each of which may beconfigured, as described herein, to distribute incoming light acrosseach respective waveguide, for output toward the eye 4. Light exits anoutput surface 1300, 1302, 1304, 1306, 1308 of the image injectiondevices 1200, 1202, 1204, 1206, 1208 and is injected into acorresponding input surface 1382, 1384, 1386, 1388, 1390 of thewaveguides 1182, 1184, 1186, 1188, 1190. In some embodiments, the eachof the input surfaces 1382, 1384, 1386, 1388, 1390 may be an edge of acorresponding waveguide, or may be part of a major surface of thecorresponding waveguide (that is, one of the waveguide surfaces directlyfacing the world 1144 or the viewer's eye 4). In some embodiments, asingle beam of light (e.g. a collimated beam) may be injected into eachwaveguide to output an entire field of cloned collimated beams that aredirected toward the eye 4 at particular angles (and amounts ofdivergence) corresponding to the depth plane associated with aparticular waveguide. In some embodiments, a single one of the imageinjection devices 1200, 1202, 1204, 1206, 1208 may be associated withand inject light into a plurality (e.g., three) of the waveguides 1182,1184, 1186, 1188, 1190.

In some embodiments, the image injection devices 1200, 1202, 1204, 1206,1208 are discrete displays that each produce image information forinjection into a corresponding waveguide 1182, 1184, 1186, 1188, 1190,respectively. In some other embodiments, the image injection devices1200, 1202, 1204, 1206, 1208 are the output ends of a single multiplexeddisplay which may, e.g., pipe image information via one or more opticalconduits (such as fiber optic cables) to each of the image injectiondevices 1200, 1202, 1204, 1206, 1208. It will be appreciated that theimage information provided by the image injection devices 1200, 1202,1204, 1206, 1208 may include light of different wavelengths, or colors(e.g., different component colors, as discussed herein).

In some embodiments, the light injected into the waveguides 1182, 1184,1186, 1188, 1190 is provided by a light projector system 2000, whichcomprises a light module 2040, which may include a light emitter, suchas a light emitting diode (LED). The light from the light module 2040may be directed to and modified by a light modulator 2030, e.g., aspatial light modulator, via a beam splitter 2050. The light modulator2030 may be configured to change the perceived intensity of the lightinjected into the waveguides 1182, 1184, 1186, 1188, 1190. Examples ofspatial light modulators include liquid crystal displays (LCD) includinga liquid crystal on silicon (LCOS) displays.

In some embodiments, the display system 1000 may be a scanning fiberdisplay comprising one or more scanning fibers configured to projectlight in various patterns (e.g., raster scan, spiral scan, Lissajouspatterns, etc.) into one or more waveguides 1182, 1184, 1186, 1188, 1190and ultimately to the eye 4 of the viewer. In some embodiments, theillustrated image injection devices 1200, 1202, 1204, 1206, 1208 mayschematically represent a single scanning fiber or a bundles of scanningfibers configured to inject light into one or a plurality of thewaveguides 1182, 1184, 1186, 1188, 1190. In some other embodiments, theillustrated image injection devices 1200, 1202, 1204, 1206, 1208 mayschematically represent a plurality of scanning fibers or a plurality ofbundles of scanning, fibers each of which are configured to inject lightinto an associated one of the waveguides 1182, 1184, 1186, 1188, 1190.It will be appreciated that the one or more optical fibers may beconfigured to transmit light from the light module 2040 to the one ormore waveguides 1182, 1184, 1186, 1188, 1190. It will be appreciatedthat one or more intervening optical structures may be provided betweenthe scanning fiber, or fibers, and the one or more waveguides 1182,1184, 1186, 1188, 1190 to, e.g., redirect light exiting the scanningfiber into the one or more waveguides 1182, 1184, 1186, 1188, 1190.

A controller 1210 controls the operation of one or more of the stackedwaveguide assembly 1178, including operation of the image injectiondevices 1200, 1202, 1204, 1206, 1208, the light source 2040, and thelight modulator 2030. In some embodiments, the controller 1210 is partof the local data processing module 70. The controller 1210 includesprogramming (e.g., instructions in a non-transitory medium) thatregulates the timing and provision of image information to thewaveguides 1182, 1184, 1186, 1188, 1190 according to, e.g., any of thevarious schemes disclosed herein. In some embodiments, the controllermay be a single integral device, or a distributed system connected bywired or wireless communication channels. The controller 1210 may bepart of the processing modules 70 or 72 (FIG. 1) in some embodiments.

With continued reference to FIG. 6, the waveguides 1182, 1184, 1186,1188, 1190 may be configured to propagate light within each respectivewaveguide by total internal reflection (TIR). The waveguides 1182, 1184,1186, 1188, 1190 may each be planar or have another shape (e.g.,curved), with major top and bottom surfaces and edges extending betweenthose major top and bottom surfaces. In the illustrated configuration,the waveguides 1182, 1184, 1186, 1188, 1190 may each include outcouplingoptical elements 1282, 1284, 1286, 1288, 1290 that are configured toextract light out of a waveguide by redirecting the light, propagatingwithin each respective waveguide, out of the waveguide to output imageinformation to the eye 4. Extracted light may also be referred to asoutcoupled light and the outcoupling optical elements light may also bereferred to light extracting optical elements. An extracted beam oflight is outputted by the waveguide at locations at which the lightpropagating in the waveguide strikes a light extracting optical element.The outcoupling optical elements 1282, 1284, 1286, 1288, 1290 may, forexample, be gratings, including diffractive optical features, asdiscussed further herein. While illustrated disposed at the bottom majorsurfaces of the waveguides 1182, 1184, 1186, 1188, 1190 for ease ofdescription and drawing clarity, in some embodiments, the outcouplingoptical elements 1282, 1284, 1286, 1288, 1290 may be disposed at the topand/or bottom major surfaces, and/or may be disposed directly in thevolume of the waveguides 1182, 1184, 1186, 1188, 1190, as discussedfurther herein. In some embodiments, the outcoupling optical elements1282, 1284, 1286, 1288, 1290 may be formed in a layer of material thatis attached to a transparent substrate to form the waveguides 1182,1184, 1186, 1188, 1190. In some other embodiments, the waveguides 1182,1184, 1186, 1188, 1190 may be a monolithic piece of material and theoutcoupling optical elements 1282, 1284, 1286, 1288, 1290 may be formedon a surface and/or in the interior of that piece of material.

With continued reference to FIG. 6, as discussed herein, each waveguide1182, 1184, 1186, 1188, 1190 is configured to output light to form animage corresponding to a particular depth plane. For example, thewaveguide 1182 nearest the eye may be configured to deliver collimatedlight, as injected into such waveguide 1182, to the eye 4. Thecollimated light may be representative of the optical infinity focalplane. The next waveguide up 1184 may be configured to send outcollimated light which passes through the first lens 1192 (e.g., anegative lens) before it can reach the eye 4; such first lens 1192 maybe configured to create a slight convex wavefront curvature so that theeye/brain interprets light coming from that next waveguide up 1184 ascoming from a first focal plane closer inward toward the eye 4 fromoptical infinity. Similarly, the third up waveguide 1186 passes itsoutput light through both the first 1192 and second 1194 lenses beforereaching the eye 4; the combined optical power of the first 1192 andsecond 1194 lenses may be configured to create another incrementalamount of wavefront curvature so that the eye/brain interprets lightcoming from the third waveguide 1186 as coming from a second focal planethat is even closer inward toward the person from optical infinity thanwas light from the next waveguide up 1184.

The other waveguide layers 1188, 1190 and lenses 1196, 1198 aresimilarly configured, with the highest waveguide 1190 in the stacksending its output through all of the lenses between it and the eye foran aggregate focal power representative of the closest focal plane tothe person. To compensate for the stack of lenses 1198, 1196, 1194, 1192when viewing/interpreting light coming from the world 1144 on the otherside of the stacked waveguide assembly 1178, a compensating lens layer1180 may be disposed at the top of the stack to compensate for theaggregate power of the lens stack 1198, 1196, 1194, 1192 below. Such aconfiguration provides as many perceived focal planes as there areavailable waveguide/lens pairings. Both the outcoupling optical elementsof the waveguides and the focusing aspects of the lenses may be static(i.e., not dynamic or electro-active). In some alternative embodiments,either or both may be dynamic using electro-active features.

In some embodiments, two or more of the waveguides 1182, 1184, 1186,1188, 1190 may have the same associated depth plane. For example,multiple waveguides 1182, 1184, 1186, 1188, 1190 may be configured tooutput images set to the same depth plane, or multiple subsets of thewaveguides 1182, 1184, 1186, 1188, 1190 may be configured to outputimages set to the same plurality of depth planes, with one set for eachdepth plane. This can provide advantages for forming a tiled image toprovide an expanded field of view at those depth planes.

With continued reference to FIG. 6, the outcoupling optical elements1282, 1284, 1286, 1288, 1290 may be configured to both redirect lightout of their respective waveguides and to output this light with theappropriate amount of divergence or collimation for a particular depthplane associated with the waveguide. As a result, waveguides havingdifferent associated depth planes may have different configurations ofoutcoupling optical elements 1282, 1284, 1286, 1288, 1290, which outputlight with a different amount of divergence depending on the associateddepth plane. In some embodiments, the light extracting optical elements1282, 1284, 1286, 1288, 1290 may be volumetric or surface features,which may be configured to output light at specific angles. For example,the light extracting optical elements 1282, 1284, 1286, 1288, 1290 maybe volume holograms, surface holograms, and/or diffraction gratings. Insome embodiments, the features 1198, 1196, 1194, 1192 may not be lenses;rather, they may simply be spacers (e.g., cladding layers and/orstructures for forming air gaps).

In some embodiments, the outcoupling optical elements 1282, 1284, 1286,1288, 1290 are diffractive features that form a diffraction pattern, or“diffractive optical element” (also referred to herein as a “DOE”).Preferably, the DOE's have a sufficiently low diffraction efficiency (aratio of diffracted beam intensity to the incident beam intensity) sothat only a portion of the light of the beam is deflected away towardthe eye 4 with each intersection of the DOE, while the rest continues tomove through a waveguide via total internal reflection. The lightcarrying the image information is thus divided into a number of relatedexit beams that exit the waveguide at a multiplicity of locations andthe result is a fairly uniform pattern of exit emission toward the eye 4for this particular collimated beam bouncing around within a waveguide.

In some embodiments, one or more DOEs may be switchable between “on”states in which they actively diffract, and “off” states in which theydo not significantly diffract. For instance, a switchable DOE maycomprise a layer of polymer dispersed liquid crystal, in whichmicrodroplets comprise a diffraction pattern in a host medium, and therefractive index of the microdroplets may be switched to substantiallymatch the refractive index of the host material (in which case thepattern does not appreciably diffract incident light) or themicrodroplet may be switched to an index that does not match that of thehost medium (in which case the pattern actively diffracts incidentlight).

In some embodiments, a camera assembly 500 (e.g., a digital camera,including visible light and infrared light cameras) may be provided tocapture images of the eye 4 and/or tissue around the eye 4 to, e.g.,detect user inputs and/or to monitor the physiological state of theuser. As used herein, a camera may be any image capture device. In someembodiments, the camera assembly 500 may include an image capture deviceand a light source to project light (e.g., infrared light) to the eye,which may then be reflected by the eye and detected by the image capturedevice. In some embodiments, the camera assembly 500 may be attached tothe frame 64 (FIG. 2) and may be in electrical communication with theprocessing modules 70 and/or 72, which may process image informationfrom the camera assembly 500 to make various determinations regarding,e.g., the physiological state of the user, as discussed herein. It willbe appreciated that information regarding the physiological state ofuser may be used to determine the behavioral or emotional state of theuser. Examples of such information include movements of the user and/orfacial expressions of the user. The behavioral or emotional state of theuser may then be triangulated with collected environmental and/orvirtual content data so as to determine relationships between thebehavioral or emotional state, physiological state, and environmental orvirtual content data. In some embodiments, one camera assembly 500 maybe utilized for each eye, to separately monitor each eye.

With reference now to FIG. 7, an example of exit beams outputted by awaveguide is shown. One waveguide is illustrated, but it will beappreciated that other waveguides in the waveguide assembly 1178 (FIG.6) may function similarly, where the waveguide assembly 1178 includesmultiple waveguides. Light 400 is injected into the waveguide 1182 atthe input surface 1382 of the waveguide 1182 and propagates within thewaveguide 1182 by TIR. At points where the light 400 impinges on the DOE1282, a portion of the light exits the waveguide as exit beams 402. Theexit beams 402 are illustrated as substantially parallel but, asdiscussed herein, they may also be redirected to propagate to the eye 4at an angle (e.g., forming divergent exit beams), depending on the depthplane associated with the waveguide 1182. It will be appreciated thatsubstantially parallel exit beams may be indicative of a waveguide withoutcoupling optical elements that outcouple light to form images thatappear to be set on a depth plane at a large distance (e.g., opticalinfinity) from the eye 4. Other waveguides or other sets of outcouplingoptical elements may output an exit beam pattern that is more divergent,which would require the eye 4 to accommodate to a closer distance tobring it into focus on the retina and would be interpreted by the brainas light from a distance closer to the eye 4 than optical infinity.

In some embodiments, a full color image may be formed at each depthplane by overlaying images in each of the component colors, e.g., threeor more component colors. FIG. 8 illustrates an example of a stackedwaveguide assembly in which each depth plane includes images formedusing multiple different component colors. The illustrated embodimentshows depth planes 14 a-14 f, although more or fewer depths are alsocontemplated. Each depth plane may have three component color imagesassociated with it: a first image of a first color, G; a second image ofa second color, R; and a third image of a third color, B. Differentdepth planes are indicated in the figure by different numbers fordiopters (dpt) following the letters G, R, and B. Just as examples, thenumbers following each of these letters indicate diopters (1/m), orinverse distance of the depth plane from a viewer, and each box in thefigures represents an individual component color image. In someembodiments, to account for differences in the eye's focusing of lightof different wavelengths, the exact placement of the depth planes fordifferent component colors may vary. For example, different componentcolor images for a given depth plane may be placed on depth planescorresponding to different distances from the user. Such an arrangementmay increase visual acuity and user comfort and/or may decreasechromatic aberrations.

In some embodiments, light of each component color may be outputted by asingle dedicated waveguide and, consequently, each depth plane may havemultiple waveguides associated with it. In such embodiments, each box inthe figures including the letters G, R, or B may be understood torepresent an individual waveguide, and three waveguides may be providedper depth plane where three component color images are provided perdepth plane. While the waveguides associated with each depth plane areshown adjacent to one another in this drawing for ease of description,it will be appreciated that, in a physical device, the waveguides mayall be arranged in a stack with one waveguide per level. In some otherembodiments, multiple component colors may be outputted by the samewaveguide, such that, e.g., only a single waveguide may be provided perdepth plane.

With continued reference to FIG. 8, in some embodiments, G is the colorgreen, R is the color red, and B is the color blue. In some otherembodiments, other colors associated with other wavelengths of light,including magenta and cyan, may be used in addition to or may replaceone or more of red, green, or blue. In some embodiments, features 198,196, 194, and 192 may be active or passive optical filters configured toblock or selectively light from the ambient environment to the viewer'seyes.

It will be appreciated that references to a given color of lightthroughout this disclosure will be understood to encompass light of oneor more wavelengths within a range of wavelengths of light that areperceived by a viewer as being of that given color. For example, redlight may include light of one or more wavelengths in the range of about620-780 nm, green light may include light of one or more wavelengths inthe range of about 492-577 nm, and blue light may include light of oneor more wavelengths in the range of about 435-493 nm.

In some embodiments, the light source 2040 (FIG. 6) may be configured toemit light of one or more wavelengths outside the visual perceptionrange of the viewer, for example, infrared and/or ultravioletwavelengths. In addition, the incoupling, outcoupling, and other lightredirecting structures of the waveguides of the display 1000 may beconfigured to direct and emit this light out of the display towards theuser's eye 4, e.g., for imaging and/or user stimulation applications.

With reference now to FIG. 9A, in some embodiments, light impinging on awaveguide may need to be redirected to incouple that light into thewaveguide. An incoupling optical element may be used to redirect andincouple the light into its corresponding waveguide. FIG. 9A illustratesa cross-sectional side view of an example of a plurality or set 1200 ofstacked waveguides that each includes an incoupling optical element. Thewaveguides may each be configured to output light of one or moredifferent wavelengths, or one or more different ranges of wavelengths.It will be appreciated that the stack 1200 may correspond to the stack1178 (FIG. 6) and the illustrated waveguides of the stack 1200 maycorrespond to part of the plurality of waveguides 1182, 1184, 1186,1188, 1190, except that light from one or more of the image injectiondevices 1200, 1202, 1204, 1206, 1208 is injected into the waveguidesfrom a position that requires light to be redirected for incoupling.

The illustrated set 1200 of stacked waveguides includes waveguides 1210,1220, and 1230. Each waveguide includes an associated incoupling opticalelement (which may also be referred to as a light input area on thewaveguide), with, e.g., incoupling optical element 1212 disposed on amajor surface (e.g., an upper major surface) of waveguide 1210,incoupling optical element 1224 disposed on a major surface (e.g., anupper major surface) of waveguide 1220, and incoupling optical element1232 disposed on a major surface (e.g., an upper major surface) ofwaveguide 1230. In some embodiments, one or more of the incouplingoptical elements 1212, 1222, 1232 may be disposed on the bottom majorsurface of the respective waveguide 1210, 1220, 1230 (particularly wherethe one or more incoupling optical elements are reflective, deflectingoptical elements). As illustrated, the incoupling optical elements 1212,1222, 1232 may be disposed on the upper major surface of theirrespective waveguide 1210, 1220, 1230 (or the top of the next lowerwaveguide), particularly where those incoupling optical elements aretransmissive, deflecting optical elements. In some embodiments, theincoupling optical elements 1212, 1222, 1232 may be disposed in the bodyof the respective waveguide 1210, 1220, 1230. In some embodiments, asdiscussed herein, the incoupling optical elements 1212, 1222, 1232 arewavelength selective, such that they selectively redirect one or morewavelengths of light, while transmitting other wavelengths of light.While illustrated on one side or corner of their respective waveguide1210, 1220, 1230, it will be appreciated that the incoupling opticalelements 1212, 1222, 1232 may be disposed in other areas of theirrespective waveguide 1210, 1220, 1230 in some embodiments.

As illustrated, the incoupling optical elements 1212, 1222, 1232 may belaterally offset from one another. In some embodiments, each incouplingoptical element may be offset such that it receives light without thatlight passing through another incoupling optical element. For example,each incoupling optical element 1212, 1222, 1232 may be configured toreceive light from a different image injection device 1200, 1202, 1204,1206, and 1208 as shown in FIG. 6, and may be separated (e.g., laterallyspaced apart) from other incoupling optical elements 1212, 1222, 1232such that it substantially does not receive light from the other ones ofthe incoupling optical elements 1212, 1222, 1232.

Each waveguide also includes associated light distributing elements,with, e.g., light distributing elements 1214 disposed on a major surface(e.g., a top major surface) of waveguide 1210, light distributingelements 1224 disposed on a major surface (e.g., a top major surface) ofwaveguide 1220, and light distributing elements 1234 disposed on a majorsurface (e.g., a top major surface) of waveguide 1230. In some otherembodiments, the light distributing elements 1214, 1224, 1234, may bedisposed on a bottom major surface of associated waveguides 1210, 1220,1230, respectively. In some other embodiments, the light distributingelements 1214, 1224, 1234, may be disposed on both top and bottom majorsurface of associated waveguides 1210, 1220, 1230, respectively; or thelight distributing elements 1214, 1224, 1234, may be disposed ondifferent ones of the top and bottom major surfaces in differentassociated waveguides 1210, 1220, 1230, respectively.

The waveguides 1210, 1220, 1230 may be spaced apart and separated by,e.g., gas, liquid, and/or solid layers of material. For example, asillustrated, layer 1218 a may separate waveguides 1210 and 1220; andlayer 1218 b may separate waveguides 1220 and 1230. In some embodiments,the layers 1218 a and 1218 b are formed of low refractive indexmaterials (that is, materials having a lower refractive index than thematerial forming the immediately adjacent one of waveguides 1210, 1220,1230). Preferably, the refractive index of the material forming thelayers 1218 a, 1218 b is 0.05 or more, or 0.10 or more less than therefractive index of the material forming the waveguides 1210, 1220,1230. Advantageously, the lower refractive index layers 1218 a, 1218 bmay function as cladding layers that facilitate total internalreflection (TIR) of light through the waveguides 1210, 1220, 1230 (e.g.,TIR between the top and bottom major surfaces of each waveguide). Insome embodiments, the layers 1218 a, 1218 b are formed of air. While notillustrated, it will be appreciated that the top and bottom of theillustrated set 1200 of waveguides may include immediately neighboringcladding layers.

Preferably, for ease of manufacturing and other considerations, thematerial forming the waveguides 1210, 1220, 1230 are similar or thesame, and the material forming the layers 1218 a, 1218 b are similar orthe same. In some embodiments, the material forming the waveguides 1210,1220, 1230 may be different between one or more waveguides, and/or thematerial forming the layers 1218 a, 1218 b may be different, while stillholding to the various refractive index relationships noted above.

With continued reference to FIG. 9A, light rays 1240, 1242, 1244 areincident on the set 1200 of waveguides. It will be appreciated that thelight rays 1240, 1242, 1244 may be injected into the waveguides 1210,1220, 1230 by one or more image injection devices 1200, 1202, 1204,1206, 1208 (FIG. 6).

In some embodiments, the light rays 1240, 1242, 1244 have differentproperties, e.g., different wavelengths or different ranges ofwavelengths, which may correspond to different colors. The incouplingoptical elements 1212, 122, 1232 each deflect the incident light suchthat the light propagates through a respective one of the waveguides1210, 1220, 1230 by TIR.

For example, incoupling optical element 1212 may be configured todeflect ray 1240, which has a first wavelength or range of wavelengths.Similarly, the transmitted ray 1242 impinges on and is deflected by theincoupling optical element 1222, which is configured to deflect light ofa second wavelength or range of wavelengths. Likewise, the ray 1244 isdeflected by the incoupling optical element 1232, which is configured toselectively deflect light of third wavelength or range of wavelengths.

With continued reference to FIG. 9A, the deflected light rays 1240,1242, 1244 are deflected so that they propagate through a correspondingwaveguide 1210, 1220, 1230; that is, the incoupling optical elements1212, 1222, 1232 of each waveguide deflects light into thatcorresponding waveguide 1210, 1220, 1230 to incouple light into thatcorresponding waveguide. The light rays 1240, 1242, 1244 are deflectedat angles that cause the light to propagate through the respectivewaveguide 1210, 1220, 1230 by TIR. The light rays 1240, 1242, 1244propagate through the respective waveguide 1210, 1220, 1230 by TIR untilimpinging on the waveguide's corresponding light distributing elements1214, 1224, 1234.

With reference now to FIG. 9B, a perspective view of an example of theplurality of stacked waveguides of FIG. 9A is illustrated. As notedabove, the incoupled light rays 1240, 1242, 1244, are deflected by theincoupling optical elements 1212, 1222, 1232, respectively, and thenpropagate by TIR within the waveguides 1210, 1220, 1230, respectively.The light rays 1240, 1242, 1244 then impinge on the light distributingelements 1214, 1224, 1234, respectively. The light distributing elements1214, 1224, 1234 deflect the light rays 1240, 1242, 1244 so that theypropagate towards the outcoupling optical elements 1250, 1252, 1254,respectively.

In some embodiments, the light distributing elements 1214, 1224, 1234are orthogonal pupil expanders (OPE's). In some embodiments, the OPE'sboth deflect or distribute light to the outcoupling optical elements1250, 1252, 1254 and also increase the beam or spot size of this lightas it propagates to the outcoupling optical elements. In someembodiments, e.g., where the beam size is already of a desired size, thelight distributing elements 1214, 1224, 1234 may be omitted and theincoupling optical elements 1212, 1222, 1232 may be configured todeflect light directly to the outcoupling optical elements 1250, 1252,1254. For example, with reference to FIG. 9A, the light distributingelements 1214, 1224, 1234 may be replaced with outcoupling opticalelements 1250, 1252, 1254, respectively. In some embodiments, theoutcoupling optical elements 1250, 1252, 1254 are exit pupils (EP's) orexit pupil expanders (EPE's) that direct light in a viewer's eye 4 (FIG.7).

Accordingly, with reference to FIGS. 9A and 9B, in some embodiments, theset 1200 of waveguides includes waveguides 1210, 1220, 1230; incouplingoptical elements 1212, 1222, 1232; light distributing elements (e.g.,OPE's) 1214, 1224, 1234; and outcoupling optical elements (e.g., EP's)1250, 1252, 1254 for each component color. The waveguides 1210, 1220,1230 may be stacked with an air gap/cladding layer between each one. Theincoupling optical elements 1212, 1222, 1232 redirect or deflectincident light (with different incoupling optical elements receivinglight of different wavelengths) into its waveguide. The light thenpropagates at an angle which will result in TIR within the respectivewaveguide 1210, 1220, 1230. In the example shown, light ray 1240 (e.g.,blue light) is deflected by the first incoupling optical element 1212,and then continues to bounce down the waveguide, interacting with thelight distributing element (e.g., OPE's) 1214 and then the outcouplingoptical element (e.g., EPs) 1250, in a manner described earlier. Thelight rays 1242 and 1244 (e.g., green and red light, respectively) willpass through the waveguide 1210, with light ray 1242 impinging on andbeing deflected by incoupling optical element 1222. The light ray 1242then bounces down the waveguide 1220 via TIR, proceeding on to its lightdistributing element (e.g., OPEs) 1224 and then the outcoupling opticalelement (e.g., EP's) 1252. Finally, light ray 1244 (e.g., red light)passes through the waveguide 1220 to impinge on the light incouplingoptical elements 1232 of the waveguide 1230. The light incouplingoptical elements 1232 deflect the light ray 1244 such that the light raypropagates to light distributing element (e.g., OPEs) 1234 by TIR, andthen to the outcoupling optical element (e.g., EPs) 1254 by TIR. Theoutcoupling optical element 1254 then finally outcouples the light ray1244 to the viewer, who also receives the outcoupled light from theother waveguides 1210, 1220.

FIG. 9C illustrates a top-down plan view of an example of the pluralityof stacked waveguides of FIGS. 9A and 9B. As illustrated, the waveguides1210, 1220, 1230, along with each waveguide's associated lightdistributing element 1214, 1224, 1234 and associated outcoupling opticalelement 1250, 1252, 1254, may be vertically aligned. However, asdiscussed herein, the incoupling optical elements 1212, 1222, 1232 arenot vertically aligned; rather, the incoupling optical elements arepreferably non-overlapping (e.g., laterally spaced apart as seen in thetop-down view). As discussed further herein, this nonoverlapping spatialarrangement facilitates the injection of light from different resourcesinto different waveguides on a one-to-one basis, thereby allowing aspecific light source to be uniquely coupled to a specific waveguide. Insome embodiments, arrangements including nonoverlappingspatially-separated incoupling optical elements may be referred to as ashifted pupil system, and the in coupling optical elements within thesearrangements may correspond to sub pupils.

Bragg-Reflective or Diffractive Structures Based on Liquid Crystals

Generally, liquid crystals possess physical properties that may beintermediate between conventional fluids and solids. While liquidcrystals are fluid-like in some aspects, unlike most fluids, thearrangement of molecules within liquid crystals exhibits some structuralorder. Different types of liquid crystals include thermotropic,lyotropic, and polymeric liquid crystals. Thermotropic liquid crystalsdisclosed herein can be implemented in various physical states, e.g.,phases, including a nematic state/phase, a smectic state/phase, a chiralnematic state/phase or a chiral smectic state/phase.

As described herein, liquid crystals in a nematic state or phase canhave calamitic (rod-shaped) or discotic (disc-shaped) organic moleculesthat have relatively little positional order, while having a long-rangedirectional order with their long axes being roughly parallel. Thus, theorganic molecules may be free to flow with their center of masspositions being randomly distributed as in a liquid, while stillmaintaining their long-range directional order. In some implementations,liquid crystals in a nematic phase can be uniaxial; i.e., the liquidcrystals have one axis that is longer and preferred, with the other twobeing roughly equivalent. In other implementations, liquid crystals canbe biaxial; i.e., in addition to orienting their long axis, the liquidcrystals may also orient along a secondary axis.

As described herein, liquid crystals in a smectic state or phase canhave the organic molecules that form relatively well-defined layers thatcan slide over one another. In some implementations, liquid crystals ina smectic phase can be positionally ordered along one direction. In someimplementations, the long axes of the molecules can be oriented along adirection substantially normal to the plane of the liquid crystal layer,while in other implementations, the long axes of the molecules may betilted with respect to the direction normal to the plane of the layer.

Herein and throughout the disclosure, nematic liquid crystals arecomposed of rod-like molecules with the long axes of neighboringmolecules approximately aligned to one another. To describe thisanisotropic structure, a dimensionless unit vector n called thedirector, may be used to describe the direction of preferred orientationof the liquid crystal molecules.

Herein and throughout the disclosure, a tilt angle or a pre-tilt angle Φcan refer to an angle measured in a plane perpendicular to a majorsurface (in an x-y plane) of the liquid crystal layers or of thesubstrate, e.g., the x-z plane, and measured between an alignmentdirection and the major surface or a direction parallel to the majorsurface, e.g., the x-direction.

Herein and throughout the disclosure, an azimuthal angle or a rotationangle φ is used to describe an angle of rotation about a layer normaldirection, or an axis normal to a major surface of a liquid crystallayer, which is measured in a plane parallel to a major surface of theliquid crystal layers or of the substrate, e.g., the x-y plane, andmeasured between an alignment direction, e.g., an elongation directionor the direction of the director, and a direction parallel to the majorsurface, e.g., the y-direction.

Herein and throughout the disclosure, when an angle such as the rotationangle φ or a pre-tilt angle Φ are referred to as being substantially thesame between different regions, it will be understood that an averagealignment angles can, for example, be within about 1%, about 5% or about10% of each other although the average alignment can be larger in somecases.

Herein and throughout the specification, a duty cycle can, for example,refers to a ratio between a first lateral dimension of a first regionhaving liquid crystal molecules aligned in a first alignment direction,and the grating period of the zone having the first region. Whereapplicable, the first region corresponds to the region in which thealignment of the liquid crystals does not vary between different zones.

As describe herein, liquid crystals in a nematic state or a smecticstate can also exhibit chirality. Such liquid crystals are referred toas being in a chiral phase or a cholesteric phase. In a chiral phase ora cholesteric phase, the liquid crystals can exhibit a twisting of themolecules perpendicular to the director, with the molecular axisparallel to the director. The finite twist angle between adjacentmolecules is due to their asymmetric packing, which results inlonger-range chiral order.

As described herein, liquid crystals in a chiral smectic state or phasecan be configured such that the liquid crystal molecules have positionalordering in a layered structure, with the molecules tilted by a finiteangle with respect to the layer normal. In addition, chirality caninduce successive azimuthal twists of the liquid crystal molecules withrespect to a direction perpendicular to the layer normal from one liquidcrystal molecule to the next liquid crystal molecule in the layer normaldirection, thereby producing a spiral twisting of the molecular axisalong the layer normal.

As described herein and throughout the disclosure, a chiral structurerefers to a plurality of liquid crystal molecules in a cholesteric phasethat extend in a direction, e.g., a direction perpendicular to thedirector such as a layer depth direction, and are successively rotatedor twisted in a rotation direction, e.g., clockwise or counterclockwise.In one aspect, the directors of the liquid crystal molecules in a chiralstructure can be characterized as a helix having a helical pitch.

As described herein, liquid crystals in a cholesteric phase displayingchirality can be described as having a chiral pitch, or a helical pitch(p), which corresponds to a length in the layer depth directioncorresponding to a net rotation angle of the liquid crystal molecules ofthe chiral structures by one full rotation in the first rotationdirection. In other words, the helical pitch refers to the distance overwhich the liquid crystal molecules undergo a full 360° twist. Thehelical pitch (p) can change, e.g., when the temperature is altered orwhen other molecules are added to a liquid crystal host (an achiralliquid host material can form a chiral phase if doped with a chiralmaterial), allowing the helical pitch (p) of a given material to betuned accordingly. In some liquid crystal systems, the helical pitch isof the same order as the wavelength of visible light. As describedherein, liquid crystals displaying chirality can also be described ashaving a twist angle, or a rotation angle (ϕ), which can refer to, forexample, the relative azimuthal angular rotation between successiveliquid crystal molecules in the layer normal direction, and as having anet twist angle, or a net rotation angle, which can refer to, forexample, the relative azimuthal angular rotation between an uppermostliquid crystal molecule and a lowermost liquid crystal molecule across aspecified length, e.g., the length of a chiral structure or thethickness of the liquid crystal layer.

According to various embodiments described herein, liquid crystalshaving various states or phases as described above can be configured tooffer various desirable material properties, including, e.g.,birefringence, optical anisotropy, and manufacturability using thin-filmprocesses. For example, by changing surface conditions of liquid crystallayers and/or mixing different liquid crystal materials, gratingstructures that exhibit spatially varying diffraction properties, e.g.,gradient diffraction efficiencies, can be fabricated.

As described herein, “polymerizable liquid crystals” may refer to liquidcrystal materials that can be polymerized, e.g., in-situphotopolymerized, and may also be described herein as reactive mesogens(RM).

It will be appreciated that the liquid crystal molecules may bepolymerizable in some embodiments and, once polymerized, may form alarge network with other liquid crystal molecules. For example, theliquid crystal molecules may be linked by chemical bonds or linkingchemical species to other liquid crystal molecules. Once joinedtogether, the liquid crystal molecules may form liquid crystal domainshaving substantially the same orientations and locations as before beinglinked together. For ease of description, the term “liquid crystalmolecule” is used herein to refer to both the liquid crystal moleculesbefore polymerization and to the liquid crystal domains formed by thesemolecules after polymerization.

According to particular embodiments described herein,photo-polymerizable liquid crystal materials can be configured to formBragg-reflective or diffractive structures, e.g., a diffraction grating,whose material properties, including birefringence, chirality, and easefor multiple-coating, can be utilized to create diffraction gratingswith different material properties, e.g., birefringence, chirality, andthickness, which can result in different optical properties, e.g.,diffraction efficiency, wavelength selectivity and off-axis diffractionangle selectivity, to name a few.

It will be appreciated that, as described herein, a “transmissive” or“transparent” structure, e.g., a transparent substrate, may allow atleast some, e.g., at least 20, 30 or 50%, of an incident light, to passtherethrough. Accordingly, a transparent substrate may be a glass,sapphire or a polymeric substrate in some embodiments. In contrast, a“reflective” structure, e.g., a reflective substrate, may reflect atleast some, e.g., at least 20, 30, 50, 70, 90% or more of the incidentlight, to reflect therefrom.

Optical properties of a grating are determined by the physicalstructures of the grating (e.g., the periodicity, the depth, and theduty cycle), as well as material properties of the grating (e.g.,refractive index, absorption, and birefringence). When liquid crystalsare used, optical properties of the grating can be controlled bycontrolling, e.g., molecular orientation or distribution of the liquidcrystal materials. For example, by varying molecular orientation ordistribution of the liquid crystal material across the grating area, thegrating may exhibit graded diffraction efficiencies. Such approaches aredescribed in the following, in reference to the figures.

Cholesteric Liquid Crystal Diffraction Grating (CLCG)

As described supra in reference to FIGS. 6 and 7, display systemsaccording to various embodiments described herein may include opticalelements, e.g., incoupling optical elements, outcoupling opticalelements, and light distributing elements, which may include diffractiongratings. For example, as described above in reference to FIG. 7, light400 that is injected into the waveguide 1182 at the input surface 1382of the waveguide 1182 propagates within the waveguide 1182 by totalinternal reflection (TIR). At points where the light 400 impinges on theout-coupling optical element 1282, a portion of the light exits thewaveguide as exit beams 402. In some implementations, any of the opticalelements 1182, 1282, or 1382 can be configured as a diffraction grating.

Efficient light in-coupling into (or out-coupling from) the waveguide1182 can be a challenge in designing a waveguide-based see-throughdisplays, e.g., for virtual/augmented/mixed display applications. Forthese and other applications, it is desirable to have the diffractiongrating formed of a material whose structure is configurable to optimizevarious optical properties, including diffraction properties. Thedesirable diffraction properties include, among other properties,polarization selectivity, spectral selectivity, angular selectivity,high spectral bandwidth and high diffraction efficiencies, among otherproperties. To address these and other needs, in various embodimentsdisclosed herein, the optical element 1282 is configured as acholesteric liquid crystal diffraction grating (CLCG). As describedinfra, CLCGs according to various embodiments can be configured tooptimize, among other things, polarization selectivity, bandwidth, phaseprofile, spatial variation of diffraction properties, spectralselectivity and high diffraction efficiencies.

In the following, various embodiments of CLCGs configured as areflective liquid crystal diffraction grating comprising cholestericliquid crystals (CLC) optimized for various optical properties aredescribed. Generally, diffraction gratings have a periodic structure,which splits and diffracts light into several beams travelling indifferent directions. The directions of these beams depend, among otherthings, on the period of the periodic structure and the wavelength ofthe light. To optimize certain optical properties, e.g., diffractionefficiencies, for certain applications such as outcoupling opticalelement 1282 (FIGS. 6, 7), various material properties of the CLC can beoptimized as described infra.

As described supra, liquid crystal molecules of a cholesteric liquidcrystal (CLC) layer in a chiral (nematic) phase or a cholesteric phaseis characterized by a plurality of liquid crystal molecules that arearranged to have successive azimuthal twists of the director as afunction of position in the film in a normal direction, or a depthdirection, of the liquid crystal layer. As described herein, the liquidcrystal molecules that arranged to have the successive azimuthal twistsare collectively referred to herein as a chiral structure. As describedherein, an angle (ϕ) of azimuthal twist or rotation is described as theangle between the directors the liquid crystal molecules, as describedsupra, relative to a direction parallel to the layer normal. Thespatially varying director of the liquid crystal molecules of a chiralstructure can be described as forming a helical pattern in which thehelical pitch (p) is defined as the distance (e.g., in the layer normaldirection of the liquid crystal layer) over which the director hasrotated by 3600, as described above. As described herein, a CLC layerconfigured as a diffraction grating has a lateral dimension by which themolecular structures of the liquid crystals periodically repeat in alateral direction normal to the depth direction. This periodicity in thelateral direction is referred to as a grating period (A).

According to various embodiments described herein, a diffraction gratingcomprises a cholesteric liquid crystal (CLC) layer comprising aplurality of chiral structures, wherein each chiral structure comprisesa plurality of liquid crystal molecules that extend in a layer depthdirection by at least a helical pitch and are successively rotated in afirst rotation direction. The helical pitch is a length in the layerdepth direction corresponding to a net rotation angle of the liquidcrystal molecules of the chiral structures by one full rotation in thefirst rotation direction. The arrangements of the liquid crystalmolecules of the chiral structures vary periodically in a lateraldirection perpendicular to the layer depth direction

FIG. 10 illustrates a cross-sectional side view of a cholesteric liquidcrystal (CLC) layer 1004 comprising a plurality of uniform chiralstructures, according to embodiments. The CLC 1004 comprises a CLC layer1008 comprising liquid crystal molecules arranged as a plurality ofchiral structures 1012-1, 1012-2, . . . 1012-i, wherein each chiralstructure comprises a plurality of liquid crystal molecules, where isany suitable integer greater than 2. For example, the chiral structure1012-1 comprises a plurality of liquid crystal molecules 1012-1-1,1012-1-2, . . . 1012-1-j that are arranged to extend in a layer normaldirection, e.g., the z-direction in the illustrated embodiment, where jis any suitable integer greater than 2. The liquid crystal molecules ofeach chiral structure are successively rotated in a first rotationdirection. In the illustrated embodiment, the liquid crystal moleculesare successively rotated in a clockwise direction when viewing in apositive direction of the z-axis (i.e., the direction of the axisarrow), or the direction of propagation of the incident light beams1016-L, 1016-R. For example, in the illustrated embodiment, the liquidcrystal molecules 1012-1-1, 1012-1-2, . . . 1012-1-j of the chiralstructure 1012-1 are successively rotated by rotation angles ϕ₁, ϕ₂, . .. ϕ_(j), relative to, e.g., the positive x-direction. In the illustratedembodiment, for illustrative purposes, the plurality of liquid crystalmolecules of each of the chiral structures 1012-1, 1012-2, . . . 1012-ibetween opposing ends in the z-direction are rotated by one fullrotation or turn, such that the net rotation angle of the liquid crystalmolecules is about 360°. As a result, the chiral structures 1012-1,1012-2, . . . 1012-i have a length L in the z-direction that is the sameas the helical pitch p. However, embodiments are not so limited, and thechiral structures 1012-1, 1012-2, . . . 1012-i can have any number offull rotations greater than or less than 1, any suitable net rotationangle that is lower or higher than 360°, and/or any suitable length L inthe z-direction that is shorter or longer than the helical pitch p. Forexample, in various embodiments described herein, the number of fullturns of the chiral structures can be between 1 and 3, between 2 and 4,between 3 and 5, between 4 and 6, between 5 and 7, between 6 and 8,between 7 and 9, or between 8 and 10, among other numbers.

Still referring to FIG. 10, the successive rotation angles betweenadjacent liquid crystal molecules in the z-direction, ϕ₁, ϕ₂, . . .ϕ_(j), can be the same according to some embodiments, or be differentaccording to some other embodiments. By way of illustration, in theillustrated embodiment, the length of the chiral structures 1012-1,1012-2, . . . 1012-i is about p and the net rotation angle is 360°, suchthat adjacent liquid crystal molecules in the z-direction are rotated byabout 360°/(m−1), where m is the number of liquid crystal molecules in achiral structure. For example, for illustrative purposes, each of thechiral structure 1012-1, 1012-2, . . . 1012-i has 13 liquid crystalmolecules, such that adjacent liquid crystal molecules in thez-direction are rotated with respect to each other by about 300. Ofcourse, chiral structures in various embodiments can have any suitablenumber of liquid crystal molecules.

Thus, still referring to FIG. 10, the chiral structures that areadjacent in a lateral direction, e.g., x-direction, have similarlyarranged liquid crystal molecules. In the illustrated embodiment, thechiral structures 1012-1, 1012-2, . . . 1012-i are similarly configuredsuch that liquid crystal molecules of the different chiral structuresthat are at about the same depth, e.g., the liquid crystal moleculesclosest to the light-incident surface 1004S, have the same rotationangle, as well as successive rotation angles of successive liquidcrystal molecules at about the same depth, as well as the net rotationangle of the liquid crystal molecules of each chiral structure.

In the following, the CLC layer 1004 illustrated in FIG. 10 is furtherdescribed in operation, according to embodiments. As described, the CLClayer 1004 comprises the chiral structures 1012-1, 1012-2, . . . 1012-ihaving a uniform arrangement in a lateral direction, e.g., x-direction.In operation, when incident light having a combination of light beamshaving left-handed circular polarization and light beams havingright-handed circular polarization are incident on the surface 1004S ofthe CLC layer 1008, by Bragg-reflection or diffraction, light with oneof the circular polarization handedness is reflected by the CLC layer1004, while light with the opposite polarization handedness istransmitted through the CLC layer 1008 without substantial interference.As described herein and throughout the disclosure, the handedness isdefined as viewed in the direction of propagation. According toembodiments, when the direction of polarization, or handedness of thepolarization, of the light beams 1016-L, 1016-R is matched such that itand has the same direction of rotation as the liquid crystal moleculesof the chiral structures 1012-1, 1012-2, . . . 1012-i, the incidentlight is reflected. As illustrated, incident on the surface 1004S arelight beams 1016-L having left-handed circular polarization and lightbeams 1016-R having a right-handed circular polarization. In theillustrated embodiment, the liquid crystal molecules of the chiralstructures 1012-1, 1012-2, . . . 1012-i are rotated in a clockwisedirection successively in the direction in which incident light beams1016-L, 1016-R travel, i.e., positive x-direction, which is the samerotation direction as the light teams 1016-R having right-handedcircular polarization. As a result, the light beams 1016-R havingright-handed circular polarization are substantially reflected, whereasthe light beams 1016-L having left-handed circular polarization aresubstantially transmitted through the CLC layer 1004.

Without being bound to any theory, under a Bragg-reflection ordiffraction condition, the wavelength of the incident light (k) may beproportional to the mean or average refractive index (n) of a CLC layerand to the helical pitch (p), and can be expressed as satisfying thefollowing condition under some circumstances:

λ≈np  [1]

In addition, the bandwidth (Δλ) of Bragg-reflecting or diffractingwavelengths may be proportional to the birefringence Δn (e.g., thedifference in refractive index between different polarizations of light)of CLC layer 1004 and to the helical pitch (p), and can be expressed assatisfying the following condition under some circumstances:

Δλ=Δn*p  [2]

In various embodiments described herein, the bandwidth Δλ is about 60nm, about 80 nm or about 100 nm.

According to various embodiments, a peak reflected intensity within avisible wavelength range between, e.g., about 390 nm and about 700 nm,or within a near infrared wavelength range between, e.g., about 700 nmand about 2500 nm, can exceed about 60%, about 70%, about 80% or about90%. In addition, according to various embodiments, the full width athalf maximum (FWHM) can be less than about 100 nm, less than about 70nm, less than about 50 nm or less than about 20 nm.

FIG. 11 illustrates a cross-sectional side view of a CLC grating (CLCG)1150 having differently arranged chiral structures in a lateraldirection, e.g., varying twist angles in a lateral direction, accordingto embodiments. Similar to the CLC layer 1004 of FIG. 10, thediffraction grating 1150 comprises a cholesteric liquid crystal (CLC)layer 1158 comprising liquid crystal molecules arranged as a pluralityof chiral structures 1162-1, 1162-2, . . . 1162-i, wherein each chiralstructure comprises a plurality of liquid crystal molecules. Forexample, the chiral structure 1162-1 comprises a plurality of liquidcrystal molecules 1162-1-1, 1162-1-2, . . . 1162-1-j that are arrangedto extend in a layer normal direction, represented as z-direction in theillustrated embodiment. The liquid crystal molecules of each chiralstructure are successively rotated in a first rotation direction in asimilar manner as described with respect to FIG. 10. In addition,various other parameters of the chiral structures including the lengthL, the number of full rotations made by the liquid crystal molecules andthe number of liquid crystal molecules per chiral structure are similarto the chiral structures described above with respect to FIG. 10.

In contrast to the illustrated embodiment of FIG. 10, however, in theillustrated embodiment of FIG. 11, the chiral structures that areadjacent in a lateral direction, e.g., x-direction, have differentlyarranged liquid crystal molecules. The chiral structures 1162-1, 1162-2,. . . 1162-i are differently configured in the x-direction such that theliquid crystal molecules of the different chiral structures at about thesame depth have different rotation angles. For example, in theillustrated embodiment, the liquid crystal molecules 1162-1-1, 1162-2-1,. . . 1162-i-1, that are closest to the incident surface 1158S, of thechiral structures 1162-1, 1162-2, . . . 1162-i, respectively, aresuccessively rotated by rotation angles ϕ1, ϕ2, . . . ϕi in the positivex-axis direction relative to, e.g., positive x-direction. In theillustrated embodiment, the net rotation angle of the liquid crystalmolecules 1162-1-1, 1162-2-1, . . . 1162-i-1, that are closest to theincident surface 1158S across a lateral length A, which corresponds to aperiod of the diffraction grating 1150, is a rotation angle of about180°. In addition, liquid crystal molecules of different chiralstructures that are disposed at about the same depth level are rotatedby about the same rotation angle relative to respective surface-mostliquid crystal molecules.

Still referring to FIG. 11, the successive rotation angles ϕ₁, ϕ₂, . . .ϕ_(i) of liquid crystal molecules that are at the same depth levelacross the period Λ in the x-direction can be the same according to someembodiments, or be different according to some other embodiments. In theillustrated embodiment, for the period Λ, when the net rotation angle is360° as in the illustrated embodiment, adjacent liquid crystal moleculesin the x-direction are rotated by about 360°/(m−1), where m is thenumber of liquid crystal molecules spanned by a period Λ in thex-direction. For example, for illustrative purposes, there are 7 liquidcrystal molecules that span across the period Λ, such that adjacentliquid crystal molecules at the same vertical level in the x-directionare rotated with respect to each other by about 30°. Of course, chiralstructures in various embodiments can have any suitable number of liquidcrystal molecules.

It will be appreciated that, for illustrating purposes, the CLC layer1158 is illustrated to have only one period Λ. Of course, embodimentsare not so limited, and the CLC layer 1158 can have any suitable numberof periods that is determined by the lateral dimension of the CLCG inthe x-direction.

As illustrated by the CLCG 1150, when the chiral structures in a lateraldirection, e.g., x-direction, are differently arranged, e.g.,successively rotated, the successively rotated chiral structures induceshifts in the relative phases of the reflected light along thex-direction. This is illustrated with respect to graph 1170, which plotsthe phase change ϕ resulting from the chiral structures that aresuccessively rotated by rotation angles ϕ₁, ϕ₂, . . . ϕ_(i) in thex-axis direction in one period Λ. Without being bound to any theory, therelative phase difference (Δϕ) of reflected light 1018 can be expressedas Δϕ(x)=(2πx/Λ), where x is the position along the lateral directionand A is the period. The bandwidth can be expressed as Δλ≈Δn*p.

Referring back to FIGS. 10-11 and Eqs. [1] and [2], according to variousembodiments, the Bragg-reflected wavelength can be varied by varying thehelical pitch p of the chiral structures. In various embodiments,without being bound to any theory, the helical pitch p can be varied byincreasing or decreasing helical twisting power (HTP), which refers tothe ability of a chiral compound to induce the rotation or twist anglesas described above. The HTP can in turn be varied by changing the amountof chiral compound relative to the amount of non-chiral compound. Invarious embodiments, by mixing a chiral compound chemically and/ormechanically with a non-chiral compound, e.g., a nematic compound, theBragg-reflection wavelength and thus the color can be varied based on aninverse relationship between the relative fraction of the chiralcompound and the helical pitch. In various embodiments disclosed herein,the ratio of the amount of chiral compound to the amount of nonchiralcompound can be about 20:1, 10:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3,1:4, 1:5, 1:10 or about 1:20 by weight.

In the description supra with respect to FIGS. 10 and 11, the incidentlight beams 1016-L, 1016-R are illustrated as being propagating in adirection parallel to the layer normal, e.g., in the z-direction. Forvarious applications, however, e.g., as described above with respect toFIGS. 6 and 7, light propagating within the waveguide 1182, e.g.,propagating by total internal reflection (TIR), impinges on theoutcoupling optical elements 1282, 1284, 1286, 1288, 1290, e.g.,diffraction gratings, at an off-axis angle. The diffraction gratingsdescribed herein can be configured to maximize bandwidth and diffractionefficiency for such configurations, as described below.

In the description supra with respect to FIGS. 10 and 11, the liquidcrystal molecules are illustrated to not be pre-tilted. Embodiment arenot so limited, however, and the liquid crystal molecules can have apre-tilt angle D, relative to a direction parallel to a major surface ofthe CLCG, e.g., relative to the x-y plane that is between about +/−60degrees and about +/−90 degrees or between about +/−65 degrees and about+/−85 degrees, for instance about +/−75 degree; between about +/−35degrees and about +/−65 degrees or between about +/−40 degrees and about+/−60 degrees, for instance about +/−50 degrees; between about +/−10degrees and about +/−40 degrees or between about +/−15 degrees and about+/−35 degrees, for instance about +/−25 degrees, according to someembodiments. According to some other embodiments, the pre-tilt angle Φcan be between about ±15 degrees or between about ±10 degrees or betweenabout ±5, e.g., 0 degrees.

CLCGs Configured for High Bandwidth Reflection at Off-Axis IncidentAngle

FIG. 12 illustrates a cross-sectional side view of a CLC layer 1158configured for high bandwidth of reflection at an off-axis incidentangle, according to embodiments. As described herein, an off-axisincident angle refers an angle of incidence θ_(inc) of an incident beam1216 with respect to the direction of layer normal (e.g., z-direction inFIG. 12) that has a non-zero value, resulting in a Bragg-reflected beam1220 at a reflection angle θ. Under some circumstances, the reflectionangle can be varied to a limited extent by varying a λ/Λ. Without beinglimited by any theory, under some circumstances, off-axis reflection canbe described based on the following relationship:

n·sin(θ)=λ/Λ+sin(θ_(inc)),  [3]

where θ_(inc) is the incident angle relative to the direction of layernormal, θ is the reflection angle relative to the direction of layernormal and n is a reflective index of a medium in which the reflectedbeam propagates. When the CLC layer 1158 is illuminated with theincident beam 1216 at an off-axis angle, the reflection spectrum may beshifted toward shorter wavelengths. According to various embodimentsdisclosed herein, the ratio λ/Λ can have a value between 0.5 and 0.8,between 0.6 and 0.9, between 0.7 and 1.0, between 0.8 and 1.1, between0.9 and 1.2, between 1.0 and 1.6, between 1.1 and 1.5, or between 1.2and 1.4.

Without being bound to any theory, the off-axis angle at which the CLClayer 1158 is configured to Bragg-reflect with high efficiency can alsodepend on the helical pitch p of the chiral structures.

FIGS. 13A and 13B illustrate cross-sectional side views of CLC layersconfigured for reflection at off-axis incident angles, according toembodiments. Referring to FIG. 13A, a first cholesteric liquid crystal(CLC) layer 1358A comprises a first plurality of chiral structureshaving a first helical pitch (p₁). The first CLC layer 1358A has a firsthelical pitch p1 such that Bragg-reflection is at a maximum when a firstincident light beam 1316A is directed to an incident surface of the CLClayer 1358A at a first off-axis angle θ_(inc,1), which results in afirst reflected light beam 1320A at a first reflection angle θ₁. Asillustrated, the CLC layer 1358A is further configured to have a firstrange 1324A of off-axis incident angles in which relatively highdiffraction efficiency can be obtained. The first range 1324A cancorrespond to a range of off-axis incident angles outside of which theintensity of the first reflected light beam 1320A falls off by morethan, e.g., l/e. For example, the first range 1324A can have values of,θ_(inc,1)±3°, θ_(inc,1)±5°, θ_(inc,1)±7°, θ_(inc,1)±10° orθ_(inc,1)±20°.

Referring to FIG. 13B, a second cholesteric liquid crystal (CLC) layer1358B different from the first CLC layer 1358A comprising a secondplurality of chiral structures having a second helical pitch (p₂)different from the first helical pitch p₁ of the first CLC layer 1358Aof FIG. 13A.

As illustrated, the second CLC layer 1358B is configured such that whena second incident light beam 1316B is directed to an incident surface ofthe CLC layer 1358B at a second off-axis angle θ_(inc,2) different fromthe first off-axis angle θ_(inc1), a second reflected light beam 1320Bhaving a second reflection angle θ₂ different from the first reflectionangle θ₁ is generated As illustrated, the CLC layer 1358B is furtherconfigured to have a second range 1324B of off-axis angles, similar tothe first range 1324A described above with respect to FIG. 13A.

FIG. 13C illustrates a cross-sectional side view of a CLCG 1358including a plurality of CLC layers having different helical pitches ina stacked configuration for Bragg-reflection at a plurality of off-axisincident angles and high diffraction bandwidth, according toembodiments. The CLCG 1358 includes CLC layers 1358A, 1358B describedabove with respect to FIGS. 13A and 13B, respectively, that are formedover one another, e.g., in a stacked configuration and/or in contactwith each other, according to embodiments. Various parameters of theplurality of CLC layers 1358A, 1358B including the different helicalpitches can be improved or optimized such that the CLCG 1358 isconfigured for efficient reflection at a plurality of off-axis incidentangles and for high diffraction efficiency over a wider range ofoff-axis angles than can be obtained using only one CLC. For example, inthe illustrated embodiments, p₁ and p₂ can be selected such that theresulting first and second ranges 1324A and 1324B at least partiallyoverlap to provide high diffraction efficiency over a contiguous rangeof wavelength that includes the first and second ranges 1324A and 1324B.However, in other embodiments, p₁ and p₂ can be selected such that thefirst and second ranges 1324A and 1324B do not overlap.

In operation, the first and second CLC layers 1358A, 1358B are formedover one another such that when first and second incident light beams1316A, 1316B at first and second off-axis angles θ_(inc1), θ_(inc2), aredirected to an incident surface of the first CLC layer 1358A, the firstincident light beam 1316A is substantially reflected by the first CLClayer 1358A at a first reflection angle θ₁, while the second incidentlight beam 1358B substantially transmits through the first CLC layer1358A towards an incident surface of the second CLC layer 1358B, andsubstantially reflected by the second CLC layer 1358B at the secondreflection angle θ₂. It will be appreciated that, while not shown forclarity, the concepts described above can be extended to any suitablenumber of CLC layers.

As described herein and throughout the specification, a light beam that“substantially transmits” through a layer may refer to the light havingat least 20%, 30%, 50%, 70% or 90%, of an incident light intensityremaining as the light exits the layer. Similarly, a light beam that is“substantially reflected” by a layer may refer to the light having atleast 20, 30, 50%, 70% or 90%, of an incident light intensity remainingin the reflected light.

Still referring to FIG. 13C, in various embodiments, the liquid crystalmolecules of the first and second CLC layers 1358A, 1358B can includethe same chiral compound at different amounts, such that CLC layers1358A, 1358B have different helical twisting power (HTP), as describedsupra. For example, the second CLC layer 1358B may have a higherrelative amount of the same chiral compound compared to the first CLClayer 1358A. In some embodiments, the pitch p may be inverselyproportional to the fraction of the chiral compound relative to thetotal liquid crystal compound which includes chiral and nonchiralcompounds. However, embodiments are not so limited, and the first andsecond CLC layers 1358A, 1358B can have different chiral compounds.

In addition, in various embodiments, the liquid crystal molecules of thefirst and second CLC layers 1358A, 1358B can include the same ordifferent chiral compounds, such that the CLC layers 1358A, 1358B havedifferent ratios λ/Λ₁ and λ/Λ₂, respectively, such that the CLC layers1358A, 1358B can be configured for high diffraction efficiencies atdifferent incident angles θ_(inc1), θ_(inc2), e.g., according to Eq.[3].

Still referring to FIG. 13C, first and second CLC layers 1358A, 1358Bcan be fabricated directly on the top of each other, according to someembodiments. For example, the first CLC layer 1358A can be deposited onan alignment layer that provides alignment conditions for the first CLClayer 1358A and subsequently, the second CLC layer 1358B can bedeposited on the first CLC layer 1358B. Under these fabricationconditions, the surface of the first CLC layer 1358A can providealignment conditions for the second CLC layer 1358B. In some otherembodiments, each of the CLC layers 1358A, 1358B can be fabricated withseparate alignment layers. For example, the first CLC layer 1358A can beformed on a first alignment layer and, a second alignment layer can beformed on the first CLC layer 1358A, and the second CLC layer 1358B onthe second alignment layer. An isolation layer, e.g., a thin oxidelayer, may be formed on the first CLC layer 1358A, according to someembodiments, prior to forming the second alignment layer and/or thesecond CLC layer 1358B. In yet other embodiments, the two CLC layers1358A, 1358B can be fabricated individually on different substrates andsubsequently stacked. In various embodiments, an intermediate layer canbe formed between the two CLC layers 1358A, 1358B, e.g., to enhanceadhesion.

The concepts described above with respect to CLCGs having a plurality ofCLC layers optimized for optimum diffraction efficiency at differentoff-axis angles can be extended to other alternative embodiments. Inparticular, in some embodiments, instead of forming a plurality oflayers, a single CLC layer can be configured to have different regionsthat are optimized for optimum diffraction efficiency at differentoff-axis angles.

FIG. 14 illustrates a cross-sectional side view of a CLCG 1400 includinga single CLC layer 1404 having vertical regions with different helicalpitches along a depth direction for Bragg-reflection at a plurality ofoff-axis incident angles at different vertical regions with highdiffraction bandwidth, according to embodiments. The CLC layer 1404 hasa plurality of vertical regions having different parameters, e.g.,different helical pitches, that are optimized such that high diffractionefficiency can be obtained over a wider range of off-axis angles thancan be obtained using only one CLC layer having a uniform pitch in thedepth direction. In the illustrated embodiment, the single CLC layer1404 includes a plurality of vertical regions 1404A, 1404B, 1404C and1404D, which can have different helical pitches p₁, p₂, p₃ and p₄,respectively. Similar to as described above with respect to FIG. 13C,the helical pitches p₁, p₂, p₃ and p₄ can be selected such that theplurality of vertical regions 1404A, 1404B, 1404C and 1404D areconfigured for optimum diffraction efficiency at incident anglesθ_(incA), θ_(incB), θ_(incC) and θ_(incD), respectively, which resultsin reflected light beams at different vertical depths at correspondingreflection angles θ_(A), θ_(B), θ_(C), and θ_(D), respectively.Furthermore, as described above with respect to FIG. 13C, the CLC layer1404 is further configured to have respective ranges of off-axis anglesin which relatively high diffraction efficiency can be obtained. Ofcourse, while four vertical regions are illustrated for clarity, anysuitable number of regions can be included in the CLC layer 1404. Inaddition, different variations described above with respect to the CLCG1358 of FIG. 13C having a plurality of CLC layers can be applicable tothe CLCG 1400.

In the illustrated embodiment of FIG. 14, the values of the helicalpitches p1, p2, p3 and p4 decrease with increasing depth from anincident surface 1404S, such that a decreasing gradient in helical pitchis created in the depth direction (negative z-direction). When the rateof decrease of the helical pitch as a function of layer depth in thez-direction is uniform across the thickness of the CLC layer 1404, agraph 1408 representing a linear relationship between the depth and thehelical pitch can be obtained. However, embodiments are not so limited.For example, the helical pitches p₁, p₂, p₃ and p₄ can increase ordecrease at any depth and can change at different rates as a function oflayer depth, according to some other embodiments.

The CLC layer 1404 having a gradient in helical pitch can be fabricated,by varying, e.g., increasing or decreasing, the helical twisting power(HTP) of the liquid crystal molecules at different depths of the CLClayer. The HTP can in turn be spatially varied by changing the relativeamount of chiral compound. In various embodiments, by mixing a chiralcompound chemically and/or mechanically with a non-chiral compound,e.g., a nematic compound, at different vertical depths, the helicalpitches of the vertical regions 1404A, 1404B, 1404C and 1404D can beconfigured for optimum diffraction efficiency at different incidentangles θ_(incA), θ_(incB), θ_(incC) and θ_(incD), respectively, based onan inverse relationship between the relative fraction of the chiralcompound and the helical pitch. For example, a mixture of differentchemical components (e.g., chiral di-acrylate monomers andnematic/non-chiral mono-acrylate monomers) that undergo polymerizationprocess at different reaction rates under UV irradiation can be used.Additionally or alternatively, the HTP can be spatially varied bychanging irradiation conditions, including exposure intensity and/orexposure time, of UV irradiation at different depths of the CLC layer.The HTP can also be spatially varied by varying the pre-/post-processingof UV polymerization process including thermal treatments before, afterand/or during UV irradiation. For example, when a UV absorbing dye isadded to a mixture, an intensity gradient of the UV light at differentdepth of the CLC layer can be created. For example, due to the UVintensity gradient, the polymerization near the surface may proceed at afaster rate compared to the bottom region of the CLC layer. For example,when the cholesteric component is a di-acrylate, the probability ofbeing incorporated into the resulting polymer can be much higher, e.g.,twice as high, as the probability of nematic mono-acrylate beingincorporated in the polymer. Under some circumstances, if the overallpolymerization rate is controlled such that a depletion of the chiraldiacrylate near surface region of the CLC layer generates a di-acrylateconcentration gradient in the depth direction of the CLC layer. This inturn starts diffusion of the di-acrylate towards the surface region ofthe CLC layer. The result after complete photo-polymerization can bethat the surface region of the CLC layer contains more chiral materialand thus has a shorter helical pitch compared to the bottom region ofthe CLC layer, which contains a relatively higher amount of non-chiralcompound. Under some other circumstances, thermal treatment before/afteror during UV irradiation can be added in the polymerization process tocontrol the helical pitch gradient. Thus, by controlling the ratiobetween two different liquid crystal monomers and/or the dose of UVirradiation at different depths with or without thermal treatment, ahelical pitch gradient can be achieved along the depth direction of theCLC layer.

For some applications, it may be desirable to have certain opticalcharacteristics of a diffraction grating, such as off-angle diffractionefficiency, refractive index, wavelength selectivity, polarizationselectivity and phase selectivity, among other parameters, to vary alonga lateral direction orthogonal to the layer normal direction. Thelateral variation be desired, for example, when the grating is stackedwith a waveguide, e.g., as illustrated above with respect to FIGS. 6 and7, such that the light propagates in the lateral direction. Under suchconfiguration, however, the intensity of light may attenuate as itpropagates within the waveguide (e.g., 1182 in FIG. 7). Suchconfigurations may also be desirable, for example, to intentionally skewthe light intensity across the grating (e.g., 1282 in FIG. 7) to adaptto spatial and/or angular variation of sensing efficiencies associatedwith the human eye to maximize the user experience. Thus, there is aneed for optical elements, e.g., diffraction gratings, having spatiallyvarying optical characteristics.

FIG. 15 illustrates a cross-sectional side view of a CLCG including aCLC layer having lateral regions with different helical pitches along alateral direction for spatially varying Bragg-reflection, according toembodiments. The CLC layer 1424 has a plurality of lateral regionshaving different liquid crystal material parameters, e.g., helicalpitches, such that laterally varying properties, e.g., laterally varyingoff-axis incident angles for Bragg reflection, can be obtained. In theillustrated embodiment, the CLC layer 1424 includes a plurality oflateral regions 1424A, 1424B and 1424C each having a period Λ and havingrespective helical pitches p₁, p₂ and p₃. The helical pitches p₁, p₂ andp₃ can be selected such that the plurality of vertical regions 1424A,1424B and 1404C are configured for optimum diffraction efficiency atdifferent off-axis incident angles θ_(incA), θ_(incB) and θ_(incC)respectively, which results in reflected light beams at correspondingreflection angles θ_(A), θ_(B), and θ_(C), respectively. Furthermore, asdescribed above with respect to FIG. 13C, different lateral regions ofthe CLC layer 1424 are further configured to have similar respectiveranges of off-axis angles in which relatively high diffractionefficiency can be obtained. Of course, while three vertical regions areillustrated for clarity, any suitable number of regions can be includedin the CLC layer 1424.

In the illustrated embodiment of FIG. 15, the magnitudes of helicalpitches p₁, p₂ and p₃ can change monotonically in a lateral direction,such that a gradient in helical pitch is created. When the rate ofchange of the helical pitch in the x-direction is uniform across a widthor a length of the CLC layer 1424, a linear relationship between thelength or width and the helical pitch can be obtained, as illustrated ingraph 1428 representing a. However, embodiments are not so limited. Forexample, the helical pitches p₁, p₂ and p₃ can increase or decrease atany lateral position and can change at different rates in thex-direction along the length or width, according to various otherembodiments.

According to various embodiments, CLC layers can be fabricated to havelaterally varying diffraction characteristics by, e.g., spatiallyvarying alignment characteristics or other material properties of theliquid crystal molecules. For example, in a similar manner as describedsupra with respect to FIG. 14, e.g., by controlling the ratio betweentwo different liquid crystal monomers and/or the dose of UV irradiationin different lateral regions, a lateral helical pitch gradient can beachieved along a lateral dimension.

Waveguides Coupled with CLCG for Wavelength-Selective Light Coupling

As described supra, for various applications including incoupling andoutcoupling of light, a wave guide device can be configured to propagatelight by total internal reflection (TIR). FIG. 16 illustrates an exampleof an optical wave-guiding device 1600 comprising a waveguide 1604coupled to a CLCG 1150. The CLCG 1150 comprises liquid crystal moleculesarranged as a plurality of chiral structures in a similar manner tochiral structures 1162-1, 1162-2, . . . 1162-i described supra withrespect to FIG. 11. The waveguide 1604 is disposed over the CLCG 1150and optically coupled to the CLCG 1150. When elliptically/circularlypolarized incident light 1016-R/L has a polarization handedness whichmatches the direction of rotation of the liquid crystal molecules of thechiral structures, the incident light 1016-R/L is Bragg-reflected by theCLCG 1150 and coupled into the waveguide 1604 at an angle such that thecoupled light travels in a lateral direction (e.g., x-direction), bytotal internal reflection (TIR). Without being bound to any theory, theTIR condition can be satisfied when the diffraction angle θ is greaterthan the critical angle, θ_(C), of the waveguide. Under somecircumstances, the TIR condition can be expressed as:

sin(θ_(C))=1/n _(t)  [4]

where n_(t) is the refractive index of the waveguide 1604. According tovarious embodiments, n_(t) may be between about 1 and about 2 betweenabout 1.4 and about 1.8 or between about 1.5 and about 1.7. For example,the waveguide may comprise a polymer such as polycarbonate or a glass.

FIG. 17A illustrates a first optical wave-guiding device 1700Acomprising a first waveguide 1704A coupled to a first CLCG 1750A andconfigured to propagate light having a third wavelength λ₃ by totalinternal reflection (TIR) when θ>θ_(c3). The first CLCG 1750A has afirst period Λ₁ and a first helical pitch p₁. According to someembodiments, the first wave-guiding device 1700A may be configured forpropagating light by TIR in the visible spectrum (e.g., with wavelengthsbetween about 400 nm and 700 nm). According to some other embodiments,the first wave-guiding device 1700A may be configured for propagatinglight by TIR in the infrared spectrum (e.g., in the near-infraredportion of the spectrum with wavelengths between about 700 nm and 1400nm). As described above with respect to FIGS. 10 and 11,Bragg-reflection occurs at a wavelength expressed by Eq. [1] supra andwithin a bandwidth of wavelength Δλ expressed by Eq. [2] supra. Forexample, the first CLCG 1750A may be designed for coupling by TIR thirdincident light 1736 having a third wavelength λ₃ in one of blue color(e.g., about 450 nm), green color (e.g., about 550 nm), red color (e.g.,about 650 nm) or in the infrared. As illustrated, when Δλ is about 60nm, about 80 nm or about 100 nm, as described supra, first and secondlight 1716 and 1726 having first and second wavelengths λ₁, λ₂ aresubstantially transmitted because Eq. [1] is not satisfied for thesecolors, which are not coupled into the first waveguide 1704 because Eq.[4] is not satisfied.

FIG. 17B illustrates a second optical wave-guiding device 1700B combinedwith the first optical wave-guiding device 1700A illustrated above withrespect to FIG. 17A. The optical wave-guiding device 1700B is disposedin the optical path subsequent to the optical wave-guiding device 1700A,and comprises a second waveguide 1704B coupled to a second CLCG 1750Band configured to propagate second light 1726 having a second wavelengthλ₂ by total internal reflection (TIR) when θ>θ_(c2). The second CLCG1750B has a second period Λ₂ and a second helical pitch p₂. As describedabove with respect to FIG. 17A, first and second light 1716 and 1726having first and second wavelengths of λ₃, λ₂ are substantiallytransmitted through the first optical wave-guiding device 1700A. Of thetransmitted first and second light 1716 and 1726, the second CLCG 1750Bmay be designed for coupling by TIR the second incident light 1726having the second wavelength λ₂ in transmitted one of blue color (e.g.,about 450 nm), green color (e.g., about 550 nm), red color (e.g., about650 nm) or infrared, when θ>θ_(c2). Thus, as illustrated, when Δλ isabout 60 nm, about 80 nm or about 100 nm, as described supra, firstlight 1716 having the first wavelength λ₁ is substantially transmittedfurther through the second wave-guiding device 1700B.

FIG. 17C illustrates a third optical wave-guiding device 1700C combinedthe first and second optical wave-guiding devices 1700A and 1700Billustrated above with respect to FIG. 17B. The third opticalwave-guiding device 1700C is disposed in the optical path subsequent tothe first and second optical wave-guiding devices 1700A and 1700B, andcomprises a third waveguide 1704C coupled to a third CLCG 1750C andconfigured to propagate first light 1716 having a first wavelength λ₂ bytotal internal reflection (TIR) when θ>θ_(c1). The third CLCG 1750C hasa third period Λ₃ and a third helical pitch p₃. As described above withrespect to FIG. 17B, first light 1716 having first wavelength λ₁ issubstantially is transmitted through the first and second wave-guidingdevices 1700A and 1700B. The third CLCG 1750C may be designed forcoupling by TIR the first incident light 1716 having the firstwavelength λ₁ in transmitted one of blue color (e.g., about 450 nm),green color (e.g., about 550 nm), red color (e.g., about 650 nm) orinfrared when θ>θ_(c). Thus, as illustrated, when Δλ is about 60 nm,about 80 nm or about 100 nm, as described supra, first light 1716 havingthe first wavelength λ₁ is substantially coupled into the thirdwaveguide 1704C because Eq. [4] is satisfied.

Thus, as described above with respect to FIGS. 17A-17C, by placing oneor more of the first, second and third optical wave-guiding devices1700A, 1700B and 1700C in the same optical path, one or more of first,second and third light 1716, 1726 and 1736 having different wavelengthsλ₁, λ₂ and λ₃ can be coupled to propagate by TIR in one of first, secondand third waveguides 1704A, 1704B and 1704C, respectively. While in eachof FIGS. 17A-17C, each of the first to third optical wave-guidingdevices 1704A, 1704B and 1704C has a dedicated first to third waveguides1704A, 1704B and 1704C, respectively, and a dedicated first to thirdCLCGs 1750A, 1750B and 1750C, embodiments are not so limited. Forexample, a single waveguide can couple by TIR Bragg-reflected light froma stack of a plurality of CLCGs, as illustrated infra with respect toFIG. 18. In addition, any suitable number of optical wave-guidingdevices greater than three (or less than three) can also be combined forfurther selective coupling by Bragg-reflection.

FIG. 18 illustrates an optical wave-guiding device 1800 comprising acommon waveguide 1704 coupled to a plurality of CLCGs 1750. Theplurality of CLCGs 1750 is configured as a stack comprising the first tothird CLCGs 1750A-1750C and configured to propagate third, second andfirst light 1736, 1726 and 1716 having third, second and firstwavelengths λ₃, λ₂ and λ₃, respectively, by total internal reflection(TIR). The TIR occurs when one or more of third, second and first lights1736, 1726 and 1716, respectively, satisfies the condition θ>θ_(c3)θ>θ_(c2) and θ>θ_(c1), respectively, in a similar manner as describedabove with respect to FIGS. 17A-17C. Also in a similar manner, first,second and third CLCGs 1750A, 1750B and 1750C are configured toselectively Bragg-reflect third, second and first light 1736, 1726 and1716, respectively, when θ>θ_(c3) θ>θ_(c2) and θ>θ_(c1). Of course, anysuitable number CLCGs less than or greater than three (or less thanthree) can be stacked for further selective coupling byBragg-reflection. Thus, compared to the embodiments described above withrespect to FIGS. 17B and 17C, a more compact wave-guiding device 1800can be obtained by employing a common waveguide 1704. Also, instead ofthree distinct CLCG layers (as shown in FIG. 18), the stack of CLCGlayers could be arranged as a single (or multiple) layers having ahelical pitch gradient comprising the range from p₁ to p₃.

As described above with respect to FIGS. 17A-18, first to third CLCGs1750, 1750B, 1750C have first to third periods Λ₁, Λ₂ and Λ₃,respectively and first to third helical pitches p₁, p₂ and p₃,respectively. In various embodiments, each of the CLCGs can beconfigured such that the wavelength/period ratio λ/Λ is between about0.3 and 2.3, between about 0.8 and 1.8 or between about 1.1 and about1.5, for instance about 1.3. Alternatively, the period (A) can beconfigured to be between about 1 nm and 250 nm smaller, between about 50nm and 200 nm smaller or between about 80 nm and 170 nm smaller, thanthe respective wavelength (λ) that the CLCGs are configured for Braggreflection. For example, when λ₃, λ₂ and λ₃ are within the visiblerange, e.g., about 620 nm to about 780 nm, for instance about 650 nm(red), about 492 nm to about 577 nm, for instance 550 nm (green), andabout 435 nm to about 493 nm, for instance about 450 nm (blue),respectively, the corresponding periods Λ₁, Λ₂ and Λ₃ can be about 450nm to about 550 nm, for instance about 500 nm, about 373 nm to about 473nm, for instance about 423 nm, and about 296 nm to about 396 nm, forinstance about 346 nm, respectively. Alternatively, when λ₁, λ₂ and λ₃are in the infrared range, e.g., in the near infrared range betweenabout 750 nm to about 1400 nm, for instance about 850 nm, thecorresponding periods Λ₁, Λ₂ and Λ₃ can be about 975 nm to about 1820nm, for instance about 1105 nm. In addition, various embodiments, eachof the CLCGs can be configured such that the wavelength/helical pitchratio λ/p is between about 0.6 and 2.6, between about 1.1 and 2.1 orbetween about 1.4 and about 1.8, for instance about 1.6. Alternatively,the helical pitch (p) can be configured to be between about 50 nm and350 nm smaller, between about 100 nm and 300 nm smaller or between about140 nm and 280 nm smaller, than the respective wavelength (λ) that theCLCGs are configured for Bragg reflection. For example, when λ₁, λ₂ andλ₃ are about 620 nm to about 780 nm, for instance about 650 nm (red),about 492 nm to about 577 nm, for instance 550 nm (green), and about 435nm to about 493 nm, for instance about 450 nm (blue), respectively, thecorresponding helical pitches p₁, p₂ and p₃ can be about 350 nm to about450 nm, for instance about 400 nm, about 290 nm to about 390 nm, forinstance about 340 nm and about 230 nm to about 330 nm, for instanceabout 280 nm, respectively. Alternatively, when λ₃, λ₂ and λ₃ are in theinfrared range, e.g., in the near infrared range between about 750 nm toabout 1400 nm, for instance about 850 nm, the corresponding periods Λ₁,Λ₂ and Λ₃ can be about 1200 nm to about 2240 nm, for instance about 1360nm.

Waveguides Coupled with CLCG and a Mirror for Wavelength-Selective LightCoupling

FIG. 19 illustrates an optical wave-guiding device 1900 comprising awaveguide 1604 coupled to a CLCG 1150, similar to the opticalwave-guiding device described supra with respect to FIG. 16. Asdescribed supra with respect to FIGS. 10 and 11, in operation, when thehandedness of polarization of the elliptical/circularly polarizedincident light has the same direction of rotation as the liquid crystalmolecules of the chiral structures of the CLCG 1150, the CLCG 1150substantially reflects the incident light. As illustrated, incident onthe surface 1050S are light beams 1016-L having a left-handed circularpolarization and light beams 1016-R having a right-handed circularpolarization. In the illustrated embodiment, the liquid crystalmolecules of the chiral structures are successively rotated in aclockwise direction when viewing the direction in which incident lightbeams 1016-L, 1016-R travel, i.e., the negative z-direction, such thatthe rotation direction of the liquid crystal molecules match thehandedness of the light teams 1016-R having a right-handed circularpolarization. As a result, the light beams 1016-R having a right-handedcircular polarization are substantially reflected by the CLCG 1150,whereas the light beams 1016-L having a left-handed circularpolarization are substantially transmitted through the CLCG 1150.

For some applications, it may desirable to flip the polarizationhandedness of an elliptical or circular polarized light prior tocoupling into a wave-guiding device similar to that described above withrespect to FIG. 19. Such may the case, e.g., when the polarizationhandedness of the incident elliptical or circular polarized light doesnot match the rotation direction of the chiral structures in the CLCGsuch that the CLCG is not configured to be Bragg-reflect the light forcoupling into the waveguide, as discussed supra. For some otherapplications, it may be desirable to recycle light that is transmittedthrough the CLCG due to a lack of match between the polarizationhandedness of the incident elliptical or circular polarized light andthe rotation direction of the chiral structures in the CLCG. To addressthese and other needs, in the following, various embodiments ofwave-guiding devices employing a polarization converting reflector toaddress these needs are disclosed.

FIG. 20 illustrates an optical wave-guiding device 2000 comprising awaveguide 1150 coupled to a CLCG 1604 and a polarization convertingreflector 2004, where the CLCG 1604 is configured to receive incidentlight and the waveguide 1150 is configured to propagate lightBragg-reflected from the CLCG by total internal reflection (TIR),according to embodiments. The polarization converting reflector 2004 isconfigured such that, upon reflection therefrom, the polarizationhandedness of an incident elliptically or circularly polarized light isflipped to an opposite polarization handedness (e.g., left-handed toright-handed, or right-handed to left-handed). The wave-guiding device2000 is similar to the wave-guiding device 1900 described above withrespect to FIG. 19 except, instead of being configured to first receivean incident light beam through the waveguide 1150, the wave-guidingdevice 2000 is configured to first receive an incident light beam 2016-Lhaving, e.g., a left-handed circular polarization, through the CLCG1604. The incident light beam 2016-L has a polarization handedness thatdoes not match the rotation direction of the chiral structures in theCLCG 1604 when viewed in the propagation direction (negativez-direction) of the incident light beam 2016-L, such that it is notBragg-reflected by the CLCG 1604. As a result, the incident light beam2016-L is substantially transmitted through the CLCG 1604 andsubsequently reflected by the polarization converting reflector 2004.The reflected light beam 2016-R having, e.g., a right-handed circularpolarization, thereby becomes an incident light beam on the surface1150S of the waveguide 1150. Because of the flipped polarizationhandedness, the reflected light beam 2016-R now incident on surface1150S of the waveguide 1150 has a polarization handedness that matchesthe rotation direction of the chiral structures in the CLCG 1604 whenviewed in the propagation direction of reflected light beam 2016-R(positive z-direction), such that it is Bragg-reflected by the CLCG1604. The reflected light beam 2016-R that is reflected as furtherreflected beam 2018 reflected at an angle θ>θ_(c) relative to the layernormal direction (z-axis) couples to and travels through the waveguide1150 in a lateral direction (e.g., x-direction).

FIG. 21A illustrates the optical wave-guiding device 2000 describedabove with respect to FIG. 20 under a condition in which the incidentlight beam 2116 is linearly polarized or unpolarized, each of which canbe treated as comprising both left-handed and right-handed circularpolarization components. Under such conditions, the incident light beam2116 can be coupled into a waveguide by TIR in opposing lateraldirections. For example, similar to as described above with respect toFIG. 20, the component of the incident light beam 2116 that has apolarization handedness, e.g., left-handedness, that does not match therotation direction of the chiral structures in the CLCG 1604 issubstantially transmitted through the CLCG 1604 and subsequentlyreflected by the polarization converting reflector 2004, to be flippedin polarization handedness, e.g., flipped to right-handedness, andcoupled into and travels through the waveguide 1150 in a first lateraldirection (e.g., positive x-direction). On the other hand, similar to asdescribed above with respect to FIG. 19, the component of the incidentlight beam 2116 that has a polarization handedness, e.g.,right-handedness, that matches the rotation direction of the chiralstructures in the CLCG 1604, is substantially directly reflected by theCLCG 1604 and subsequently coupled into and travels through thewaveguide 1150 in a second lateral direction opposite the first lateraldirection (e.g., negative x-direction).

FIG. 21B illustrates the optical wave-guiding device 2000 describedabove with respect to FIG. 21A under a condition in which the incidentlight is polarized into two orthogonal elliptical or circular polarizedlight beams, e.g., light beams 1016-L having left-handed circularpolarization and light beams 1016-R having right-handed circularpolarization. Under such conditions, the incident light beams 1016-L,1016-R can be coupled into a waveguide by TIR to propagate in opposinglateral directions, in a similar manner as described with respect toFIG. 21A, supra. For example, the light beams 1016-L that has apolarization handedness, e.g., left-handedness, that does not match therotation direction of the chiral structures in the CLCG 1604 issubstantially transmitted through the CLCG 1604 and subsequentlyreflected by the polarization converting reflector 2004, to be flippedin polarization handedness, e.g., flipped to right-handedness, andcoupled into and travels through the waveguide 1150 in a first lateraldirection (e.g., positive x-direction). On the other hand, the incidentlight beam 1016-R that has a polarization handedness, e.g.,right-handedness, that matches the rotation direction of the chiralstructures in the CLCG 1604, is substantially directly reflected by theCLCG 1604 and subsequently coupled into and travels through thewaveguide 1150 in a second lateral direction opposite the first lateraldirection (e.g., negative x-direction).

FIG. 22A illustrates an optical wave-guiding device 2200 comprising acommon waveguide 2204 coupled to a plurality of CLCGs that are, e.g.,arranged as a stack, including a first CLCG 2204 having chiralstructures having a first rotation direction and a second CLCG 2208having chiral structures having a second rotation direction opposite tothe first rotation direction, according to embodiments. As describedsupra with respect to various embodiments, in operation, when thedirection of polarization direction of an incident light beam is matchedto the direction of rotation of the liquid crystal molecules of chiralstructures of a CLCG, the incident light is reflected. The illustratedoptical wave-guiding device 2200 is under a condition in which theincident light beam 2116 is linearly polarized or unpolarized. Undersuch conditions, the incident light beam 2116 can be coupled into awaveguide by TIR in both of opposing lateral directions (positive andnegative x directions). In the illustrated embodiment, when viewing inthe direction in which incident light 2116 travels, i.e., the negativez-direction, the liquid crystal molecules of the chiral structures ofthe first CLCG 2204 are successively rotated in a clockwise direction,while the liquid crystal molecules of the chiral structures of thesecond CLCG 2204 are successively rotated in the oppositecounterclockwise direction.

Still referring to FIG. 22A, the component of the elliptical or circularincident light beam 2116 that has a first polarization handedness, e.g.,right-handed polarized component, that matches the rotation direction ofthe chiral structures of the first CLCG 2204, e.g., clockwise direction,is substantially reflected by the first CLCG 2204, thereby resulting ina first reflected beam 2118A at an angle θ>θ_(c1) relative to the layernormal direction (z-axis), and couples to and travels through the commonwaveguide 2204 in a first lateral direction (e.g., positivex-direction).

Still referring to FIG. 22A, on the other hand, the component of theelliptical or circular incident light beam 2116 that has a secondpolarization handedness, e.g., left-handed polarized component, thatdoes not match the rotation direction of the chiral structures of thefirst CLCG 2204, is substantially transmitted through the first CLCG2204. After being transmitted through the first CLCG 2204, theelliptical or circular incident light beam 2116 that has the secondpolarization handedness 2116 that does match the rotation direction ofthe chiral structures of the second CLCG 2208, e.g., counter-clockwisedirection, is substantially reflected by the second CLCG 2208, therebyresulting in a second reflected beam 2118B at an angle θ>θ_(c2) relativeto the layer normal direction (z-axis), and couples to and travelsthrough the common waveguide 2204 in a second lateral direction (e.g.,negative x-direction).

FIG. 22B illustrates the optical wave-guiding device 2000 describedabove with respect to FIG. 22A under a different condition in which theincident light is polarized into two orthogonal elliptical or circularpolarized light beams, e.g., light beams 1016-L having, e.g., aleft-handed elliptical/circular polarization and light beams 1016-Rhaving, e.g., a right-handed elliptical/circular polarization. Undersuch condition, the incident light beams 1016-L, 1016-R can be coupledinto the common waveguide 2204 by TIR in opposing lateral directions, ina similar manner as described with respect to FIG. 22A, supra, forcoupling the incident light beams 1016-L, 1016-R having first and secondpolarization handedness, e.g., left-handedness and right-handedness.

The embodiments described above with respect to FIGS. 21B and 22B can beparticularly advantageous in certain applications, e.g., where differentlight signals (i.e., images) are encoded in orthogonal circularpolarizations. Under such circumstances, light can be coupled into theopposite directions (e.g., positive and negative x-directions) dependingon the polarization handedness.

FIG. 22C illustrates an optical wave-guiding device 2220 comprising acommon waveguide 2250 coupled to a plurality of CLCGs, e.g., arranged asa stack, including a first CLCG 2204 having chiral structures having afirst rotation direction and a second CLCG 2208. having chiralstructures having a second rotation direction opposite to the firstrotation direction, according to embodiments. Unlike the embodimentsdescribed with respect to FIGS. 22A and 22B, in the wave-guiding device2220, the common waveguide 2250 is interposed between the first andsecond CLCG layers 2204, 2208. For illustrative purposes, theillustrated optical wave-guiding device 2220 is under a condition inwhich the incident light beam 2116 is linearly polarized or unpolarized.Under such conditions, the incident light beam 2116 can be coupled intoa waveguide by TIR in opposing lateral directions. In the illustratedembodiment, when viewing the direction in which incident light 2116travels, i.e., the negative z-direction, the liquid crystal molecules ofthe chiral structures of the first CLCG 2204 are successively rotated ina clockwise direction, while the liquid crystal molecules of the chiralstructures of the second CLCG 2204 are successively rotated in theopposite counterclockwise direction. Of course, opposite arrangement ispossible.

Still referring to FIG. 22C, the component of the elliptical or circularincident light beam 2116 that has a first polarization handedness, e.g.,right-handed polarized component, that matches the rotation direction ofthe chiral structures of the first CLCG 2204, e.g., clockwise direction,is substantially reflected by the first CLCG 2204, thereby resulting ina first reflected beam 2118A at an angle θ>θ_(c1) relative to the layernormal direction (z-axis), which in turn reflects off of the outersurface of the first CLCG 2204, before coupling into and travelingthrough the common waveguide 2250 in a first lateral direction (e.g.,negative x-direction) by TIR.

Still referring to FIG. 22C, on the other hand, the component of theelliptical or circular incident light beam 2116 that has a secondpolarization handedness, e.g., left-handed polarized component, thatdoes not match the rotation direction of the chiral structures of thefirst CLCG 2204, e.g., clockwise direction, is substantially transmittedthrough the first CLCG 2204 and further through the common waveguide2204, and thereafter substantially reflected by the second CLCG 2208,thereby resulting in a second reflected beam 2218B at an angle θ>θ_(c2)relative to the layer normal direction (z-axis), and couples to andtravels through the common waveguide 2250 in a second lateral direction(e.g., positive x-direction) by TIR.

Cholesteric Liquid Crystal Off-Axis Mirror

As described supra with respect to various embodiments, by matching thehandedness of polarization of incident elliptically or circularlypolarized light with the direction of rotation as the liquid crystalmolecules of the chiral structures of a CLC layer, the CLC layer can beconfigured as a Bragg reflector. Furthermore, one or more CLC layershaving different helical pitches can be configured as a wave-lengthselective Bragg reflector with high bandwidth. Based on the conceptsdescribed herein with respect to various embodiments, the CLC layers canbe configured as an off-axis mirror configured to selectively reflect afirst range of wavelengths, for example, infrared wavelengths (e.g., thenear infrared), while transmitting another range of wavelengths, e.g.,visible wavelengths. In the following, applications of variousembodiments of CLC off-axis mirrors implemented in eye-tracking systemsare disclosed, according to embodiments.

FIG. 23 illustrates an example of an eye-tracking system 2300 employinga cholesteric liquid crystal reflector (CLCR), e.g., awavelength-selective CLCR 1150 configured to image an eye 302 of aviewer, according to various embodiments. Eye tracking can be a keyfeature in interactive vision or control systems including wearabledisplays, e.g., the wearable display system 200 in FIG. 2 or the systems700 described in FIGS. 24A-24H, for virtual/augmented/mixed realitydisplay applications, among other applications. To achieve good eyetracking, it may desirable to obtain images of the eye 302 at lowperspective angles, for which it may in turn be desirable to dispose aneye-tracking camera 702 b near a central position of viewer's eyes.However, such position of the camera 702 b may interfere with user'sview. Alternatively, the eye-tracking camera 702 b may be disposed to alower position or a side. However, such position of the camera mayincrease the difficulty of obtaining robust and accurate eye trackingsince the eye images are captured at a steeper angle. By configuring theCLCR 1150 to selectively reflect infrared (IR) light 2308 (e.g., havinga wavelength of 850 nm) from the eye 302 while transmitting visiblelight 2304 from the world as shown in FIG. 4, the camera 702 b can beplaced away from the user's view while capturing eye images at normal orlow perspective angles. Such configuration does not interfere withuser's view since visible light is not reflected. The same CLCR 1150 canalso be configured as an IR illumination source 2320, as illustrated. Alow perspective angle of IR illuminator can results in less occlusions,e.g., from eye lashes, which configuration allows more robust detectionof specular reflections, which can be key feature in modern eye-trackingsystems.

Still referring to FIG. 23, according to various embodiments, the CLCR1150 comprises one or more cholesteric liquid crystal (CLC) layers eachcomprising a plurality of chiral structures, wherein each chiralstructure comprises a plurality of liquid crystal molecules that extendin a layer depth direction (e.g., z-direction) and are successivelyrotated in a first rotation direction, as described supra. Thearrangements of the liquid crystal molecules of the chiral structuresvary periodically in a lateral direction perpendicular to the layerdepth direction such that the one or more CLC layers are configured tosubstantially Bragg-reflect a first incident light having a firstwavelength (λ₁) while substantially transmitting a second incident lighthaving a second wavelength (λ₂). As described elsewhere in thespecification, each of the one or more CLC layers are configured tosubstantially Bragg-reflect elliptically or circularly polarized firstand second incident light having a handedness of polarization that ismatched to the first rotation direction, when viewed in the layer depthdirection, while being configured to substantially transmit ellipticallyor circularly polarized first and second incident light having ahandedness of polarization that is opposite to the first rotationdirection, when viewed in the layer depth direction. Accordingembodiments, the arrangements of the liquid crystal molecules varyingperiodically in the lateral direction are arranged to have a period inthe lateral direction such that a ratio between the first wavelength andthe period is between about 0.5 and about 2.0. According to embodiments,the first wavelength is in the near infrared range between about 600 nmand about 1.4 μm, for instance about 850 nm and the second wavelength inis in the visible range having one or more colors as described elsewherein the specification. According to embodiments, the liquid crystalmolecules of the chiral structures are pre-tilted relative to adirection normal to the layer depth direction. As configured, the one ormore CLC layers are configured such that the first incident light isreflected at an angle (θ_(R)) relative to the layer depth direction(z-direction) exceeding about 500, about 600, about 700 or about 800degrees relative to the layer depth direction based on, e.g., Eq. [3]described supra.

Referring back to FIG. 2, the eyes of the wearer of a head mounteddisplay (HMD) (e.g., the wearable display system 200 in FIG. 2) can beimaged using a reflective off-axis Diffractive Optical Element (DOE),which may be for example, a Holographic Optical Element (HOE). Theresulting images can be used to track an eye or eyes, image the retina,reconstruct the eye shape in three dimensions, extract biometricinformation from the eye (e.g., iris identification), etc.

There are a variety of reasons why a head mounted display (HMD) mightuse information about the state of the eyes of the wearer. For example,this information can be used for estimating the gaze direction of thewearer or for biometric identification. This problem is challenging,however, because of the short distance between the HMD and the wearer'seyes. It is further complicated by the fact that gaze tracking requiresa larger field of view, while biometric identification requires arelatively high number of pixels on target on the iris. For an imagingsystem which will attempt to accomplish both of these objectives, therequirements of the two tasks are largely at odds. Finally, bothproblems are further complicated by occlusion by the eyelids andeyelashes. Embodiments of the imaging systems described herein addresssome or all of these problems. The various embodiments of the imagingsystems 700 described herein with reference to FIGS. 24A-24F can be usedwith HMD including the display devices described herein (e.g., thewearable display system 200 shown in FIG. 2 and/or the display system1000 shown in FIG. 6).

FIG. 24A schematically illustrates an example of an imaging system 700that comprises an imager 702 b which is used to view the eye 304, andwhich is mounted in proximity to the wearer's temple (e.g., on a frame64 of the wearable display system 200, FIG. 2, for example, an earstem). In other embodiments, a second imager is used for the wearer'sother eye 302 so that each eye is separately imaged. The imager 702 bcan include an infrared digital camera that is sensitive to infraredradiation. The imager 702 b is mounted so that it is facing forward (inthe direction of the wearer's vision), rather than facing backward anddirected at the eye 304 (as with the camera 500 shown in FIG. 6). Bydisposing the imager 702 b nearer the ear of the wearer, the weight ofthe imager 702 b is also nearer the ear, and the HMD may be easier towear as compared to an HMD where the imager is backward facing anddisposed nearer to the front of the HMD (e.g., close to the display 62,FIG. 2). Additionally, by placing the forward-facing imager 702 b nearthe wearer's temple, the distance from the wearer's eye 304 to theimager is roughly twice as large as compared to a backward-facing imagerdisposed near the front of the HMD (e.g., compare with the camera 500shown in FIG. 4). Since the depth of field of an image is roughlyproportional to this distance, the depth of field for the forward-facingimager 702 b is roughly twice as large as compared to a backward-facingimager. A larger depth of field for the imager 702 b can be advantageousfor imaging the eye region of wearers having large or protruding noses,brow ridges, etc.

The imager 702 b is positioned to view an inside surface 704 of anotherwise transparent optical element 706. The optical element 706 canbe a portion of the display 708 of an HMD (or a lens in a pair ofeyeglasses). The optical element 706 can be transmissive to at least10%, 20%, 30%, 40%, 50%, or more of visible light incident on theoptical element. In other embodiments, the optical element 706 need notbe transparent (e.g., in a virtual reality display). The optical element706 can comprise a CLC off-axis mirror 708. The CLC off-axis mirror 708can be a surface reflecting a first range of wavelengths while beingsubstantially transmissive to a second range of wavelengths (that isdifferent from the first range of wavelengths). The first range ofwavelengths can be in the infrared, and the second range of wavelengthscan be in the visible. For example, the CLC off-axis mirror 708 cancomprise a hot mirror, which reflects infrared light while transmittingvisible light. In such embodiments, infrared light 710 a, 712 a, 714 afrom the wearer propagates to and reflects from the optical element 706,resulting in reflected infrared light 710 b, 712 b, 714 b which can beimaged by the imager 702 b. In some embodiments, the imager 702 b can besensitive to or able to capture at least a subset (such as a non-emptysubset and/or a subset of less than all) of the first range ofwavelengths reflected by the CLC off-axis mirror 708. For example, theCLC off-axis mirror 708 may reflect infrared light in the a range of 700nm to 1.5 am, and the imager 702 b may be sensitive to or able tocapture near infrared light at wavelengths from 700 nm to 900 nm. Asanother example, the CLC off-axis mirror 708 may reflect infrared lightin the a range of 700 nm to 1.5 am, and the imager 702 b may include afilter that filters out infrared light in the range of 900 nm to 1.5 amsuch that the imager 702 b can capture near infrared light atwavelengths from 700 nm to 900 nm.

Visible light from the outside world (1144, FIG. 6) is transmittedthrough the optical element 706 and can be perceived by the wearer. Ineffect, the imaging system 700 shown in FIG. 24A acts as if there were avirtual imager 702 c directed back toward the wearer's eye 304. Thevirtual imager 702 c can image virtual infrared light 710 c, 712 c, 714c (shown as dotted lines) propagated from the wearer's eye 704 throughthe optical element 706. Although the hot mirror (or other DOE describedherein) can be disposed on the inside surface 704 of the optical element706, this is not a limitation. In other embodiments, the hot mirror orDOE can be disposed on an outside surface of the optical element 706 orwithin the optical element 706 (e.g., a volume HOE).

FIG. 24B schematically illustrates another example of the imaging system700. In this embodiment, perspective distortions may be reduced oreliminated by the use of a perspective control lens assembly 716 b(e.g., a shift lens assembly, a tilt lens assembly, or a tilt-shift lensassembly) with the imager 702 b. In some embodiments, the perspectivecontrol lens assembly 716 b may be part of the lens of the imager 702 b.The perspective control lens 716 b can be configured such that a normalto the imager 702 b is substantially parallel to a normal to the regionof the surface 704 that includes the DOE (or HOE) or hot mirror. Ineffect, the imaging system 700 shown in FIG. 24B acts as if there were avirtual imager 702 c with a virtual perspective control lens assembly716 c directed back toward the wearer's eye 304.

Additionally or alternatively, as schematically shown in FIG. 24C, theCLC off-axis mirror 708 of the optical element 706 may have, on itssurface 704, an off axis holographic mirror (OAHM), which is used toreflect light 710 a, 712 a, 714 a to facilitate viewing of the eye 304by the camera imager 702 b which captures reflected light 710 b, 712 b,714 b. The OAHM 708 may have optical power as well, in which case it canbe an off-axis volumetric diffractive optical element (OAVDOE), asschematically shown in FIG. 24D. In the example shown in FIG. 24D, theeffective location of the virtual camera 702 c is at infinity (and isnot shown in FIG. 24D).

In some embodiments, the HOE (e.g., the OAHM or OAVDOE) can be dividedinto a plurality of segments. Each of these segments can have differentoptical properties or characteristics, including, for example,reflection angles at which the segments reflect the incoming (infrared)light or optical power. The segments can be configured so that light isreflected from each segment toward the imager 702 b. As a result, theimage acquired by the imager 702 b will also be divided into acorresponding number of segments, each effectively viewing the eye froma different angle. FIG. 24E schematically illustrates an example of thedisplay system 700 having an OAHM with three segments 718 a 1, 718 a 2,718 a 3, each of which acts as a respective virtual camera 702 c 1, 702c 2, 702 c 3 imaging the eye 304 at a different angular location.

FIG. 24F schematically illustrates another example of the display system700 having an OAHM with three segments 718 a 1, 718 a 2, 718 a 3, eachhaving optical power (e.g., a segmented OAVDOE), with each segmentgenerating a virtual camera at infinity imaging the eye 304 at adifferent angular location. Although three segments are schematicallyillustrated in FIGS. 24E and 24F, this is for illustration and notlimitation. In other embodiments, two, four, five, six, seven, eight,nine, or more segments can be utilized. None, some, or all of thesesegments of the HOE can have optical power.

The three segments 718 a 1, 718 a 2, 718 a 3 are shown as spacedhorizontally across the optical element 706 in FIGS. 24E and 24F. Inother embodiments, the segments can be spaced vertically on the opticalelement 706. For example, FIG. 24G schematically shows a DOE 718 havingtwo vertically spaced segments 718 a 1 and 718 a 2, with the segment 718a 1 comprising a CLC off-axis mirror configured to reflect light backtoward the imager 702 b (which may be in the same general horizontalplane as the segment 718 a 1), and the segment 718 a 2 configured toreflect light upwards toward the imager 702 b. Similar to bifocallenses, the arrangement shown in FIG. 24G can be advantageous inallowing the imaging system 700 to use reflection imagery acquired bythe imager 702 b from the upper segment 718 a 1 when the wearer islooking forward through the upper portion of the HMD (schematicallyshown via the solid arrowed line) and to use reflection imagery from thelower segment 718 a 2 when the wearer is looking downward through thelower portion of the HMD (schematically shown via the dashed arrowedline).

A mix of horizontally spaced and vertically spaced segments can be usedin other embodiments. For example, FIG. 24H shows another example of theHOE 718 with a 3×3 array of segments each comprising a CLC off-axismirror. The imager 702 b can acquire reflection data from each of thesenine segments, which represent light rays coming from different areas ofand angular directions from the eye region. Two example light rayspropagating from the eye region to the HOE 718 and reflecting back tothe imager 702 b are shown as solid and dashed lines. The imaging system700 (or processing module 224 or 228) can analyze the reflection datafrom the plurality of segments to multiscopically calculate thethree-dimensional shape of the eye or the gaze direction (e.g., eyepose) of the eye.

Embodiments of the optical system 700 utilizing segments may havemultiple benefits. For example, the segments can be used individually,by selecting the particular segments which best suit a particular task,or they can be used collectively to multiscopically estimate thethree-dimensional shape or pose of the eye. In the former case, thisselectivity can be used to, for example, select the image of thewearer's iris which has the least occlusion by eyelids or eyelashes. Inthe latter case, the three dimensional reconstruction of the eye can beused to estimate orientation (by estimation of, for example, thelocation of the bulge of the cornea) or accommodation state (byestimation of, for example, the lens induced distortion on the apparentlocation of the pupil).

Waveguides Coupled with CLCG Optimized for Field of View

A medium having a refractive index that depends on the polarization andpropagation direction of light is referred to as being birefringent (orbirefractive). As described throughout the specification and understoodin the relevant industry, light whose polarization is perpendicular tothe optic axis of a birefringent medium is described as being affectedby an ordinary refractive index (n_(o)), light whose polarization isparallel to the optic axis of the birefringent medium is described asbeing affected by an extraordinary refractive index (n_(e)), and adifference of the refractive indices, n_(e)−n_(o), observed in thebirefringent medium material is described as having a birefringence Δn.As described herein, an average refractive index n_(L)c of abirefringent CLCG can be expressed as:

n _(LC)=½(n _(o) +n _(e))=n _(o) +Δn/2.  [5]

According to various embodiments described herein, cholesteric liquidcrystal (CLC) layers can have an average, a local, a mean, a median, amaximum or a minimum birefringence (Δn) of 0.05-0.10, 0.15-0.20,0.20-0.25, 0.25-0.30, 0.30-0.35, 0.35-0.40, 0.40-0.45, 0.45-0.50,0.50-0.55, 0.55-0.60, 0.60-0.65, 0.65-0.70, or a value within a rangedefined by any of these values.

As described herein, the phase retardation (F) of light in a materialmedium having birefringence Δn can be expressed as F=2Δnd/λ, where λ isthe wavelength of light and d is the thickness of the medium. Inaddition, diffraction efficiency (η) of a birefringent medium such as alayer having liquid crystals can be expressed as η=sin²(πΔnd/λ), whereΔn is birefringence, λ is wavelength and d is the thickness of themedium. Because the phase retardation of light propagating through thediffractive components varies with the wavelength for conventionalbirefringent media, some diffractive components including diffractiongratings may show a limited range of wavelengths, or bandwidth, withinthe visible spectrum in which diffraction efficiency is relatively high.

According to various embodiments, various CLC layers and CLCGs describedherein may be configured such that they diffract light incident thereonwith relatively high efficiency within a particular range of angles ofincidence, sometimes referred to as a range of angle of acceptance or afield-of-view (FOV). As described herein, the FOV may include a range ofangles spanning negative and positive values of angles of incidencerelative to a centerline wavelength of the FOV, outside of which thediffraction efficiency falls off by more than 10%, more than 25%, morethan 50%, more than 75%, or by a value within a range defined by any ofthese values, relative to the diffraction efficiency at the centerlinewavelength of the FOV, or relative to the diffraction efficiency at awavelength corresponding to a peak efficiency within the FOV. Otherwisestated, inside the FOV, the CLC layers and CLCGs are configured suchthat the diffraction efficiency is greater than 25%, greater than 50%,greater than 75%, greater than 90%, or a value in a range defined by anyof these values, relative to the diffraction efficiency at thecenterline wavelength of the FOV, or relative to the diffractionefficiency at a wavelength corresponding to a peak efficiency within theFOV. Having the FOV within which the diffraction efficiency isrelatively constant may be desirable, e.g., where uniform intensity ofdiffracted light is desired within the FOV.

Applicant has recognized that the FOV of the CLC layers and CLCGs can beincreased or optimized for various embodiments of waveguides coupledwith the CLC layers and CLCGs as described above, by selecting the CLClayers and CLCGs with an appropriate birefringence (see, e.g., Eq. [6]).FIG. 25 illustrates an example optical wave-guiding device 2500optimized for relatively high FOV, according to embodiments. The opticalwave-guiding device 2500 comprises a waveguide 1604 coupled to a CLCG1150. Similar to various embodiments described herein, the CLCG 1150comprises liquid crystal molecules arranged as a plurality of chiralstructures in a similar manner to chiral structures 1162-1, 1162-2, . .. 1162-i described supra, e.g., with respect to FIG. 11. The waveguide1604 is disposed over the CLCG 1150 and optically coupled thereto.

When elliptically/circularly polarized incident light 2504 having aright/left (R/L) handedness is incident on the waveguide 1604 at anangle θ_(inc) relative to a layer normal of the waveguide 1604, theincident light 2504 is coupled into the waveguide 1604 as light 2508,which becomes incident on the CLCG 1150 at an angle θ_(inc WG) relativeto the layer normal. The light 2508 coupled into the waveguide 1604,when it has a polarization handedness (R/L) which matches the directionof rotation of the liquid crystal molecules of the chiral structures inthe CLCG 1150, the light 2508 is Bragg-reflected by the CLCG 1150 intolight 2512 having an angle θ_(LC) relative to the layer normal. Thereflected light 2512 is subsequently coupled back into the waveguide1604 as light 2516 at an angle θ_(WG) relative to the layer normal, suchthat the light 2516 travels in a lateral direction (e.g., x-direction),under total internal reflection (TIR). Without being bound to anytheory, the TIR condition can be satisfied when the angle θ_(WG) isgreater than a critical angle.

Without being bound to any theory, the range of propagation angles maybe limited by the material index of the propagating medium as:

${1 - \frac{\lambda}{\Lambda}} < {\sin \mspace{14mu} \theta_{inc}} < {n - \frac{\lambda}{\Lambda}}$

When the incident angle is symmetric |sin θ_(inc)|<(n−1)/2, and theminimum refractive index for a given FOV satisfies n>2 sin θ_(inc)+1.This condition may also be valid in a CLCG layer as light diffracts andpropagates through the layer. Since the CLCG layers is birefringent, thepropagating light experiences the average index of the LC material,n_(LC). Assuming (without requiring) n_(o) to be fixed, the minimumbirefringence Δn is related to the angle of incidence of the FOV as:

Δn>2(2 sin θ_(inc) −n _(o)+1)  [6]

The LC material of the CLCG layer can be selected based on Eq. [6] toprovide a desired FOV. The FOV may have angular ranges exceeding 20°,30°, 36°, 40°, 44°, 50° or an angular range in a range of angles definedby any of these values, when the liquid crystal molecules within theCLCG 1150 are configured according to Eq. [6]. For example, thefollowing FOV incident angles can be associated with average indices as:10°: n_(LC) 1.35, 15°: n_(LC)>1.52, 18°: n_(LC)>1.62, 20°: n_(LC)>1.68,22°: n_(LC)>1.75, and 25°: n_(LC)>1.85. As another example, when n_(LC)is between about 1.35 and about 1.85, the full FOV may be between about200 and about 500, or may exceed 500.

In various embodiments, the above disclosed values of FOV can beobtained when the waveguide 1604 is formed of a suitable material havingan index of refraction n_(t) between about 1 and about 2 between about1.4 and about 1.8 or between about 1.5 and about 1.7. For example, thewaveguide may comprise a polymer such as polycarbonate or a glass.

Waveguides Coupled With CLCG Configured as Outcoupling Optical Elements

As described above with respect to FIGS. 9B and 9C, various embodimentsof display devices disclosed herein include outcoupling optical elements1250, 1252, 1254, which may be configured as exit pupil expanders(EPE's) that direct light in a viewer's eye 4 (FIG. 7). In variousembodiments described herein, various optical components such as lenses,mirrors and gratings may be configured to be specific to a certain lightpropagation direction and or to certain polarization of light, e.g.,right-handed or left-handed circular polarized light. As describedherein, in various embodiments, CLC layers and CLCGs comprise aplurality of chiral structures, where each chiral structure comprises aplurality of liquid crystal molecules that extend in a layer depthdirection by at least a helical pitch and are successively rotated in arotation direction. The CLC layers or CLCGs can advantageously beconfigured to substantially Bragg-reflect elliptically or circularlypolarized light having a handedness of polarization that is matched tothe rotation direction of the liquid crystal molecules, while beingconfigured to substantially transmit elliptically or circularlypolarized light having a handedness of polarization that is opposite tothe rotation direction of the liquid crystal molecules. Based on theseproperties of the CLC layers and CLCGs, various embodiments of displaydevices disclosed herein have optical elements 1250, 1252, 1254comprising one or more CLC layers or CLCGs.

FIG. 26 illustrates an example optical wave-guiding device 2600configured as an outcoupling optical element, such as an EPE, accordingto embodiments. The wave-guiding device 2600 comprises a waveguide 1604coupled to a CLCG 1150 and configured to propagate light by totalinternal reflection (TIR). Similar to various embodiments describedherein, the CLCG 1150 comprises liquid crystal molecules arranged as aplurality of chiral structures in a similar manner to chiral structures1162-1, 1162-2, . . . 1162-i described supra with respect to, e.g., FIG.11.

Still referring to FIG. 26, the CLCG 1150 coupled to the waveguide 1604may represent any one of the outcoupling optical elements 1250, 1252,1254 coupled to a respective one of the waveguides 1210, 1220, 1230, asillustrated in FIGS. 9B, 9C except, unlike the outcoupling opticalelements 1250, 1252, 1254 that are formed on light-exiting sides of therespective waveguides 1210, 1220, 1230, the CLCG 1150 is formed on theside opposite to the light exiting side of the waveguide 1604. Thus,according to some embodiments, each of the outcoupling optical elements1250, 1252, 1254 and a corresponding one of the incoupling opticalelements 1212, 1222, 1232 are formed on opposite sides of acorresponding one of the waveguides 1210, 1220, 1230. In operation,light in-coupled by, e.g., the incoupling optical elements 1212, 1222,1232 (FIGS. 9A-9C), propagate in a layer in-plane direction, by TIRwithin the waveguides 1210, 1220, 1230 (FIGS. 9A-9C), respectively. Theincoupled light may then impinge on the light distributing elements1214, 1224, 1234 (FIGS. 9A-9C) when present, which may deflect the lightso that it propagates towards the outcoupling optical elements 1250,1252, 1254. The light approaching the outcoupling optical elements 1250,1252, 1254 may be represented by light 2604 in in FIG. 26. Uponimpinging on the CLCG 1150, at least some of the light 2604 may bediffracted by the CLCG 1150 as diffracted light 2608 which may be, e.g.,directed into a viewer's eye 4 (FIG. 7).

Still referring to FIG. 26, the liquid crystal molecules of theillustrated CLCG 1150 are successively rotated in a rotation direction,and arrangements of the liquid crystal molecules of the chiralstructures vary periodically in a lateral direction perpendicular to thelayer depth direction. Because of the rotational arrangement of theliquid crystal molecules, when the light 2604 is anelliptically/circularly polarized light having a polarizationhandedness, e.g., one of left-handedness or right-handedness, whichmatches the direction of rotation of the liquid crystal molecules of thechiral structures, the light 2604 is Bragg-reflected by the CLCG 1150.That is, the rotational arrangement of the liquid crystal molecules inthe CLCG 1150 is such that, the CLCG 1150 selectively Bragg reflectslight having one handedness while non-Bragg reflecting or transmittinglight having the opposite handedness. In addition, because Braggreflection occurs under the diffraction condition, the Bragg reflectedlight 2608 is unidirectional (e.g., most of the light is directed towardone direction at outcoupling, such as the direction indicated by thearrows 2608 in FIG. 26). The outcoupled light can preserve a uniformpolarization state, which corresponds to the chirality of the CLCmaterial. Thus, when configured as an optical outcoupling element, theCLCG 1150 serves as a polarizer and a unidirectional reflector, whichallows for efficient integration with other optical components withinvarious the display systems described herein. For example, the opticalelement 2600 can be used as an exit-pupil expander in waveguide-based ARdisplays to project virtual images with a controlled polarization statein a single direction.

ADDITIONAL ASPECTS

In a 1^(st) aspect, a diffraction grating comprises a cholesteric liquidcrystal (CLC) layer comprising a plurality of chiral structures, whereineach chiral structure comprises a plurality of liquid crystal moleculesthat extend in a layer depth direction by at least a helical pitch andare successively rotated in a first rotation direction. The helicalpitch is a length in the layer depth direction corresponding to a netrotation angle of the liquid crystal molecules of the chiral structuresby one full rotation in the first rotation direction. Arrangements ofthe liquid crystal molecules of the chiral structures vary periodicallyin a lateral direction perpendicular to the layer depth direction.

In a 2^(nd) aspect, in the diffraction grating of the 1^(st) aspect,each chiral structure comprises at least three calamitic liquid crystalmolecules that are elongated along different elongation directions.

In a 3^(rd) aspect, in the diffraction grating of any one of the 1^(st)to 2^(nd) aspects, the CLC layer is configured to substantiallyBragg-reflect elliptically or circularly polarized light having ahandedness of polarization that is matched to the first rotationdirection, when viewed in the layer normal direction, while beingconfigured to substantially transmit elliptically or circularlypolarized light having a handedness of polarization that is opposite tothe first rotation direction, when viewed in the layer depth direction.

In a 4^(th) aspect, in the diffraction grating of any one of the 1^(st)to 3^(rd) aspects, the arrangements of the liquid crystal moleculesvarying periodically in the lateral direction are such that the liquidcrystal molecules of successively laterally adjacent chiral structuresat about the same depth in the layer depth direction are successivelyrotated in a second rotation direction by 360°/n, where n is an integer.

In a 5^(th) aspect, in the diffraction grating of any one of the 1^(st)to 4^(th) aspects, the arrangements of the liquid crystal moleculesvarying periodically in the lateral direction are such that ellipticallyor circularly polarized light that is Bragg-reflected by the laterallyadjacent chiral structures is phase-shifted by an angle that isproportional to the angle of rotation in the second rotation directionbetween the laterally adjacent chiral structures.

In a 6^(th) aspect, in the diffraction grating of any one of 1^(st) to5^(th) aspects, the chiral structures have substantially the samehelical pitch.

In a 7^(th) aspect, in the diffraction grating of any of the 1^(st) to4^(th) aspects, the chiral structures comprise a first plurality ofchiral structures each comprising a plurality of first liquid crystalmolecules that extend in a layer depth direction by at least a firsthelical pitch and are successively rotated in the first rotationdirection, and a second plurality of chiral structures each comprising aplurality of second liquid crystal molecules that extend in the layerdepth direction by at least a second helical pitch and are successivelyrotated in the first rotation direction. The first helical pitch and thesecond helical pitch are such that the first chiral structures and thesecond chiral structures are configured to Bragg-reflect light havingdifferent off-axis incident angles.

In an 8^(th) aspect, in the diffraction grating of the 7^(th) aspect,the first plurality of chiral structures are formed in a firstcholesteric liquid crystal (CLC) layer, and wherein the second pluralityof chiral structures are formed in a second cholesteric liquid crystal(CLC) layer formed over the first CLC layer and stacked in the layerdepth direction.

In a 9^(th) aspect, in the diffraction grating of the 7^(th) aspect, thefirst plurality of chiral structures are formed in a first region of thecholesteric liquid crystal (CLC) layer, and wherein the second pluralityof chiral structures are formed in a second region of the cholestericliquid crystal (CLC) layer formed over the first region in the layerdepth direction.

In a 10^(th) aspect, in the diffraction grating of the 7^(th) aspect,the first plurality of chiral structures are formed in a first region ofthe cholesteric liquid crystal (CLC) layer, and the second plurality ofchiral structures are formed in a second region of the cholestericliquid crystal (CLC) layer, wherein the first and second regions arelaterally adjacent regions in the lateral direction.

In an 11^(th) aspect, in the diffraction grating of any of the 7^(th) to10^(th) aspects, the one or both of first and second chiral structuresform a gradient in the helical pitch in one or both of the layer depthdirection and the lateral direction.

In a 12^(th) aspect, a wave-guiding device comprises one or morecholesteric liquid crystal (CLC) layers each comprising a plurality ofchiral structures, wherein each chiral structure comprises a pluralityof liquid crystal molecules that extend in a layer depth direction andare successively rotated in a first rotation direction. Arrangements ofthe liquid crystal molecules of the chiral structures vary periodicallyin a lateral direction perpendicular to the layer depth direction suchthat the one or more CLC layers are configured to Bragg-reflect incidentlight. The wave-guiding device additionally comprises one or morewaveguides formed over the one or more CLC layers and configured tooptically couple Bragg-reflected light from the one or more CLC layerssuch that the Bragg-reflected light travels in a lateral directionperpendicular to the layer depth direction by total internal reflection(TIR). The one or more CLC layers and the one or more waveguides areconfigured to be in the same optical path.

In a 13^(th) aspect, in the wave-guiding device of the 12^(th) aspect,each of the plurality of chiral structures extend in a layer depthdirection by at least a helical pitch, wherein the helical pitch is alength in the layer depth direction corresponding to a net rotationangle of the liquid crystal molecules of the chiral structures by onefull rotation in the first rotation direction.

In a 14^(th) aspect, the wave-guiding device of any one of the 12^(th)to 13^(th) aspects comprises a plurality of CLC layers, wherein each oneof the CLC layers has differently arranged chiral structures that areconfigured to selectively Bragg-reflect incident light having awavelength different than the other ones of the CLC layers and at aBragg-reflection angle different than the other ones of the CLC layers.

In a 15^(th) aspect, in the wave-guiding device of any one of the12^(th) to 14^(th) aspects, the periodically varying lateralarrangements of the liquid crystal layers are characterized by a period,wherein each one of the CLC layers has a different period than the otherones of the CLC layers.

In a 16^(th) aspect, in the wave-guiding device of any one of the12^(th) to 15^(th) aspects, each one of the CLC layers is configured toselectively Bragg-reflect incident light having a wavelength in thevisible spectrum.

In a 17^(th) aspect, in the wave-guiding device of any one of the12^(th) to 15^(th) aspects, each one of the CLC layers is configured toselectively Bragg-reflect incident light having a wavelength in theinfrared spectrum while transmitting light having wavelength in thevisible spectrum.

In an 18^(th) aspect, the wave-guiding device of any one of the 12^(th)to 16^(th) aspects comprises a plurality of waveguides, wherein eachwaveguide is optically coupled to one of the CLC layer formed thereon.

In 19^(th) aspect, in the wave-guiding device of any one of the 12^(th)to 16^(th) aspects, the plurality of CLC layers form a stack, and asingle waveguide is optically coupled to each one of CLC layers in thestack.

In a 20^(th) aspect, the wave-guiding device of any one of the 12^(th)to 16th aspects further comprises a polarizing reflector, wherein theone or more wave guides is interposed between the one or more CLC layersand the polarizing reflector and is configured such that an ellipticallyor circularly polarized incident light that transmits through the one ormore CLC layers and further through the waveguide is reflected by thepolarizing reflector as a reflected light having an oppositepolarization handedness relative to the elliptically or circularlypolarized incident light.

In a 21^(st) aspect, the wave-guiding device of any one of the 12^(th)to 16^(th) and 20^(th) aspects comprises a first CLC layer and a secondCLC layer forming a stack with a waveguide, wherein chiral structures ofthe first CLC layer and the second CLC layer are successively rotated inopposite rotation directions.

In a 22^(nd) aspect, in the wave-guiding device of the 21^(st) aspect,the first and second CLC layers are stacked on the waveguide.

In a 23^(rd) aspect, in the wave-guiding device of the 21^(st) aspect,the first and second CLC layers are interposed by the waveguide.

In a 24^(th) aspect, in the wave-guiding device of any one of the12^(th) to 23^(rd) aspects, each of the one or more CLC layers has anaverage refractive index (n_(LC)) exceeding 1.35, wherein the n_(LC) hasa value that is an average of an ordinary refractive index (n_(o)) andan extraordinary refractive index (n_(e)).

In a 25^(th) aspect, in the wave-guiding device of the 24^(th) aspect,the one or more waveguides are configured to optically coupleBragg-reflected light from the one or more CLC layers when the incidentlight is incident on the one or more CLC layer at an incident anglerelative to the layer depth direction, wherein the incident angle iswithin a field of view (FOV), within which a diffraction efficiency isgreater than 25%, spanning at least 20°.

In a 26^(th) aspect, in the wave-guiding device of any one of 24^(th)and 25^(th) aspects, each of the one or more CLC layers has abirefringence (Δn) between about 0.05 and 0.70. In any of the 24^(th) to26^(th) aspects, any of the one or more CLC layers may have abirefringence Δn greater than 2(2 sin θ_(inc)−n_(o)+1), where θ_(inc) isan incident angle of a field of view (FOV) of the wave-guiding deviceand n_(o) is the ordinary refractive index of the one or more CLClayers.

In a 27^(th) aspect, a head-mounted display device is configured toproject light to an eye of a user to display augmented reality imagecontent. The head-mounted display device comprises a frame configured tobe supported on a head of the user. The head-mounted display deviceadditionally comprises a display disposed on the frame, where at least aportion of the display comprises one or more waveguides. The one or morewaveguides are transparent and disposed at a location in front of theuser's eye when the user wears the head-mounted display device, suchthat the transparent portion transmits light from a portion of anenvironment in front of the user to the user's eye to provide a view ofthe portion of the environment in front of the user. The display furthercomprises one or more light sources and at least one diffraction gratingconfigured to couple light from the light sources into the one or morewaveguides or to couple light out of the one or more waveguides, whereinthe at least one diffraction grating comprises a diffraction gratingaccording to any one of the 1^(st) to 11^(th) aspects.

In a 28^(th) aspect, in the head-mounted device of the 27^(th) aspect,the one or more light sources comprise a fiber scanning projector.

In a 29^(th) aspect, the head-mounted device of any one of the 27^(th)to 28^(th) aspects is configured to project light into the user's eye soas to present image content to the user on a plurality of depth planes.

In a 30^(th) aspect, a wavelength-selective cholesteric liquid crystalreflector (CLCR), comprises one or more cholesteric liquid crystal (CLC)layers each comprising a plurality of chiral structures, wherein eachchiral structure comprises a plurality of liquid crystal molecules thatextend in a layer depth direction and are successively rotated in afirst rotation direction. Arrangements of the liquid crystal moleculesof the chiral structures vary periodically in a lateral directionperpendicular to the layer depth direction such that the one or more CLClayers are configured to substantially Bragg-reflect a first incidentlight having a first wavelength while substantially transmitting asecond incident light having a second wavelength.

In a 31^(st) aspect, in the wavelength-selective CLCR of the 30^(th)aspect, each of the one or more CLC layers are configured tosubstantially Bragg-reflect elliptically or circularly polarized firstor second incident light having a handedness of polarization that ismatched to the first rotation direction, when viewed in the layer depthdirection, while being configured to substantially transmit ellipticallyor circularly polarized first or second incident light having ahandedness of polarization that is opposite to the first rotationdirection, when viewed in the layer depth direction.

In a 32^(nd) aspect, in the wavelength-selective CLCR of any one of the30^(th) to 31^(st) aspects, the arrangements of the liquid crystalmolecules varying periodically in the lateral direction are arranged tohave a period in the lateral direction such that a ratio between thefirst wavelength and the period is between about 1.1 and about 1.5.

In a 33^(rd) aspect, in the wavelength-selective CLCR of any one of the30^(th) to 32^(nd) aspects, the first wavelength is in the near infraredrange between about 750 nm and about 1400 nm and the second wavelengthin is in the visible range.

In a 34^(th) aspect, in the wavelength-selective CLCR of any of one ofthe 30^(th) to 32^(nd) aspects, the one or more CLC layers areconfigured such that the first incident light is reflected at an angleexceeding 60 degrees relative to the layer depth direction.

In a 35^(th) aspect, in the wavelength-selective CLCR of any one of the30^(th) to 32^(nd) aspects, the plurality of liquid crystal molecules ofthe chiral structures are pre-tilted relative to a direction normal tothe layer depth direction.

In a 36^(th) aspect, a head mounted display (HMD) configured to be wornon a head of a user comprises a frame comprising a pair of ear stems.The HMD additionally comprises a pair of optical elements supported bythe frame such that each of the pair of optical elements is capable ofbeing disposed forward of an eye of the user. The HMD additionallycomprises a forward-facing imager mounted to one of the pair of earstems. The HMD further comprises a cholesteric liquid crystal (CLC)off-axis mirror according to any one of 30^(th) to 35^(th) aspects. Thecholesteric liquid crystal (CLC) off-axis mirror is disposed in or onone of the pair of optical elements and configured to reflect infraredlight toward the forward-facing imager that is configured to receive theinfrared light reflected by the reflective element.

In a 37^(th) aspect, in the HMD of the 36^(th) aspect, the CLC off-axismirror comprises a plurality of CLC layers, wherein each one of the CLClayers has differently arranged chiral structures that are configured toselectively Bragg-reflect incident light having a wavelength differentthan the other ones of the CLC layers and at a Bragg-reflection angledifferent than the other ones of the CLC layers.

In a 38^(th) aspect, in the HMD of any one of the 36^(th) to 37^(th)aspects, the periodically varying lateral arrangements of the liquidcrystal layers are characterized by a period, wherein each one of theCLC layers has a different period than the other ones of the CLC layers.

In a 39^(th) aspect, in the HMD of any one of the 36^(th) to 38^(th)aspects, each one of the CLC layers is configured to selectivelyBragg-reflect incident light having a wavelength in the infraredspectrum while transmitting incident light having a wavelength in thevisible spectrum.

In a 40^(th) aspect, in the HMD of any one of the 36^(th) to 39^(th)aspects, each of the pair of optical elements is transparent to visiblelight.

In a 41^(st) aspect, in the HMD of any one of the 36^(th) to 40^(th)aspects, each of the pair of optical elements is configured to displayan image to the user.

In a 42^(nd) aspect, a wave-guiding device comprises one or morecholesteric liquid crystal (CLC) layers each comprising a plurality ofchiral structures, wherein each chiral structure comprises a pluralityof liquid crystal molecules that extend in a layer depth direction andare successively rotated in a first rotation direction, whereinarrangements of the liquid crystal molecules of the chiral structuresvary periodically in a lateral direction perpendicular to the layerdepth direction such that the one or more CLC layers are configured toBragg-reflect incident light. The wave-guiding device additionallyincludes one or more waveguides formed over the one or more CLC layersand configured to optically couple Bragg-reflected light from the one ormore CLC layers such that the Bragg-reflected light travels in a lateraldirection perpendicular to the layer depth direction by total internalreflection (TIR). The wave-guiding device is configured to have a fieldof view (FOV), within which a diffraction efficiency is greater than25%, which exceeds 200.

In a 43^(rd) aspect, in the wave-guiding device of the 42^(nd) aspect,each of the one or more CLC layers has an average refractive index(n_(LC)) exceeding 1.35, wherein the n_(LC) has a value that is anaverage of an ordinary refractive index (n_(o)) and an extraordinaryrefractive index (n_(e)).

In a 44^(th) aspect, in wave-guiding device of any one of the 42^(nd) to43^(rd) aspects, each of the one or more CLC layers has a birefringence(Δn) between about 0.05 and 0.70.

In a 45^(th) aspect, a display device comprises a waveguide and anincoupling optical element formed on the waveguide and configured toincouple light incident thereon into a first side of the waveguide,wherein the incoupling optical element and the waveguide are configuredsuch that light in-coupled into the waveguide propagates in the waveguide in an in-plane direction of the waveguide by total internalreflection (TIR). The display device additionally comprises anoutcoupling optical element formed on the waveguide and configured tooutcouple light incident thereon from the waveguide. The lightout-coupling element comprises a cholesteric liquid crystal (CLC) layercomprising a plurality of chiral structures, wherein each of the chiralstructures comprises a plurality of liquid crystal molecules that extendin a layer depth direction of the CLC layer and are successively rotatedin a first rotation direction, wherein arrangements of the liquidcrystal molecules of the chiral structures vary periodically in alateral direction perpendicular to the layer depth direction such thatthe one or more CLC layers are configured to Bragg-reflect lightincident thereon from the waveguide towards the first side.

In a 46^(th) aspect, in the display device of the 45^(th) aspect, theoutcoupling optical element and the incoupling optical element areformed on opposite sides of the waveguide.

In a 47^(th) aspect, in the display device of the 45^(th) aspect, thelight outcoupling optical element is configured to selectivelyBragg-reflect light having one of right-handed circular polarization orleft-handed circular polarization.

In a 48^(th) aspect, in the display device of the 45^(th) aspect,wherein the light outcoupling optical element is configured toselectively Bragg-reflect light having a polarization direction that isthe same direction as the first rotation direction.

In a 49^(th) aspect, in the display device of the 45^(th) aspect, thelight incoupling optical element comprises a liquid crystal layer.

In a 50^(th) aspect, in the display device of the 49^(th) aspect,wherein the liquid crystal layer comprises a second CLC layer comprisinga plurality of chiral structures, wherein each of the chiral structurescomprises a plurality of liquid crystal molecules that extend in a layerdepth direction of the second CLC layer and are successively rotated inthe first rotation direction, wherein arrangements of the liquid crystalmolecules of the chiral structures vary periodically in the lateraldirection.

ADDITIONAL CONSIDERATIONS

In the embodiments described above, augmented reality display systemsand, more particularly, spatially varying diffraction gratings aredescribed in connection with particular embodiments. It will beunderstood, however, that the principles and advantages of theembodiments can be used for any other systems, apparatus, or methodswith a need for the spatially varying diffraction grating. In theforegoing, it will be appreciated that any feature of any one of theembodiments can be combined and/or substituted with any other feature ofany other one of the embodiments.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,”“include,” “including” and the like are to be construed in an inclusivesense, as opposed to an exclusive or exhaustive sense; that is to say,in the sense of “including, but not limited to.” The word “coupled”, asgenerally used herein, refers to two or more elements that may be eitherdirectly connected, or connected by way of one or more intermediateelements. Likewise, the word “connected”, as generally used herein,refers to two or more elements that may be either directly connected, orconnected by way of one or more intermediate elements. Additionally, thewords “herein,” “above,” “below,” “infra,” “supra,” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Wherethe context permits, words in the above Detailed Description using thesingular or plural number may also include the plural or singularnumber, respectively. The word “or” in reference to a list of two ormore items, that word covers all of the following interpretations of theword: any of the items in the list, all of the items in the list, andany combination of one or more of the items in the list. In addition,the articles “a,” “an,” and “the” as used in this application and theappended claims are to be construed to mean “one or more” or “at leastone” unless specified otherwise.

As used herein, a phrase referring to “at least one of” a list of itemsrefers to any combination of those items, including single members. Asan example, “at least one of: A, B, or C” is intended to cover: A, B, C,A and B, A and C, B and C, and A, B, and C. Conjunctive language such asthe phrase “at least one of X, Y and Z,” unless specifically statedotherwise, is otherwise understood with the context as used in generalto convey that an item, term, etc. may be at least one of X, Y or Z.Thus, such conjunctive language is not generally intended to imply thatcertain embodiments require at least one of X, at least one of Y and atleast one of Z to each be present.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “may,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or whether these features,elements and/or states are included or are to be performed in anyparticular embodiment.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosure. Indeed, the novel apparatus, methods, andsystems described herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe methods and systems described herein may be made without departingfrom the spirit of the disclosure. For example, while blocks arepresented in a given arrangement, alternative embodiments may performsimilar functionalities with different components and/or circuittopologies, and some blocks may be deleted, moved, added, subdivided,combined, and/or modified. Each of these blocks may be implemented in avariety of different ways. Any suitable combination of the elements andacts of the various embodiments described above can be combined toprovide further embodiments. The various features and processesdescribed above may be implemented independently of one another, or maybe combined in various ways. No element or combinations of elements isnecessary or indispensable for all embodiments. All suitablecombinations and subcombinations of features of this disclosure areintended to fall within the scope of this disclosure.

What is claimed is:
 1. A diffraction grating comprising: a cholestericliquid crystal (CLC) layer comprising a plurality of chiral structures,wherein each chiral structure comprises a plurality of liquid crystalmolecules that extend in a layer depth direction by at least a helicalpitch and are successively rotated in a first rotation direction,wherein the helical pitch is a length in the layer depth directioncorresponding to a net rotation angle of the liquid crystal molecules ofthe chiral structures by one full rotation in the first rotationdirection, and wherein arrangements of the liquid crystal molecules ofthe chiral structures vary periodically in a lateral directionperpendicular to the layer depth direction.